Molecular dynamics simulation method for oil-water interface substitution adsorption characteristics

Abstract

The invention belongs to the technical field of oil displacement of oil fields, and particularly relates to a molecular dynamics simulation method for substitution and adsorption characteristics of an oil-water interface. The simulation method is provided through substitution and adsorption of water-soluble chemical agent molecules to oil-phase active molecules on an oil-water interface, after a water-soluble chemical agent molecular model is added to a first target cubic box, molecular dynamic equilibrium simulation is carried out on a system, and the water-soluble chemical agent molecular model is obtained. Hydrophobic groups in the water-soluble chemical agent molecular model are inserted into an oil molecule cubic box due to hydrophobicity, and hydrophilic groups are substituted and adsorbed with water molecules at an oil-water interface. Molecular dynamics simulation is carried out on the water-soluble chemical agent by adjusting the type and concentration of the water-soluble chemical agent and factors such as temperature and pressure of a simulation environment, so that the competition efficiency of the water-soluble chemical agent is improved; by analyzing the interaction between the water-soluble chemical agent and the oil-phase active component and the water, the action strength between the water-soluble chemical agent and the oil-phase active component and the water is evaluated on the microscopic level, and the action mechanism of the substitution adsorption behavior is revealed.

Description

Technical Field

[0001] The invention belongs to the technical field of oilfield flooding, and in particular relates to a molecular dynamics simulation method for substitution adsorption characteristics of an oil-water interface. Background Art

[0002] During oil extraction, the adsorption behavior at the oil-water interface plays a key role in crude oil recovery. A variety of active substances are typically present on the surface of reservoir rocks, such as natural surfactants in crude oil and injected oil displacement agents (including various surfactants, polymers, nanoparticles, etc.). These substances compete for adsorption at the oil-water-rock three-phase interface. For example, when the injected active component replaces the natural active component in crude oil and adsorbs at the oil-water interface on the rock surface, if the injected active component can be more effectively adsorbed, it can reduce the oil-water interfacial tension, making it easier for crude oil to be stripped from the rock surface, thereby increasing crude oil recovery. However, if the chemical agent replacement adsorption process at the oil-water interface is not ideal, the oil displacement agent cannot fully function, and the crude oil recovery rate will be greatly limited.

[0003] In recent years, water-soluble chemicals have been widely used in oilfields. They can effectively reduce interfacial tension and are a relatively comprehensive tertiary oil recovery technology. Surfactants are key components of chemical flooding, effectively reducing interfacial tension and improving oil recovery efficiency.

[0004] While traditional macroscopic experimental methods can reveal the characteristics of chemical flooding to a certain extent, they have significant limitations. These methods struggle to delve deeply into the microscopic mechanisms of substitutional adsorption at the oil-water interface at the atomic and molecular scales. They cannot clearly reveal the details of the interactions between water-soluble chemicals and oil-phase active components at the interface, the adsorption process, and the influencing factors. This hinders a comprehensive understanding and efficient control of substitutional adsorption behavior.

[0005] Under low interfacial tension, capillary forces are reduced, making crude oil in formation pores more accessible. Numerous field and laboratory studies have shown that single surfactant systems are significantly affected by high-temperature and high-pressure environments, resulting in poor stability in formations and even loss of their ability to enhance oil and gas recovery in reservoir environments.

[0006] Currently, the characteristics of surfactant flooding, nanoparticle flooding, and polymer flooding are primarily characterized through macroscopic experiments, such as interfacial tension measurements, oil displacement efficiency experiments, adsorption capacity determination experiments, and emulsification experiments. However, there is no simple and effective method to simulate or characterize the substitution and adsorption characteristics of chemical agents at the oil-water interface. Summary of the Invention

[0007] The technical problem to be solved by this invention is to provide a molecular dynamics simulation method for oil-water interface adsorption characteristics. The molecular dynamics simulation method is used to analyze the substitution adsorption characteristics of water-soluble chemicals at the oil-water interface from an atomic perspective. The technical solution adopted is:

[0008] A molecular dynamics simulation method for substitution adsorption characteristics of an oil-water interface comprises the following steps:

[0009] Step 1: Construct the oil phase active molecule model, oil phase inactive molecule model, water-soluble chemical agent molecule model and water molecule model;

[0010] Step 2: Construct the oil phase active molecule model and the oil phase inactive molecule model into an oil molecule cubic box, and construct the water molecule model into a water molecule cubic box;

[0011] Step 3: Splice the water molecule cubic box and the oil molecule cubic box into a water-oil-water combination model with a sandwich layer structure to obtain the first target cubic box;

[0012] Step 4: Perform molecular dynamics equilibrium simulation on the first target cubic box to make the oil-water interface reach a stable state, that is, the adsorption state of the oil-phase active components at the oil-water interface reaches dynamic equilibrium;

[0013] Step 5: Observe and analyze the adsorption behavior of oil-phase active components at the oil-water interface;

[0014] Step 6: Randomly add the constructed water-soluble chemical molecular model to the water boxes on both sides to obtain the second target cubic box. Perform molecular dynamics simulation of the system again to observe the adsorption state of the water-soluble chemical at the oil-water interface under stable conditions and the changes in the adsorption state of the oil-phase active component at the oil-water interface.

[0015] Step 7: Observe and analyze the adsorption behavior of water-soluble chemicals at the oil-water interface, and study the adsorption characteristics of water-soluble chemicals replacing the active components of the oil phase at the oil-water interface; study the replacement adsorption behavior characteristics by adjusting the type and concentration of water-soluble chemicals as well as temperature and pressure.

[0016] Preferably, the construction work is started using the modeling module of professional molecular simulation software. For the oil molecule model, asphaltene is selected as the active component and saturates are selected as the inactive component; the water molecule model adopts the classic water molecule structure model.

[0017] Preferably, the periodic boundary condition setting function in the simulation software is used to arrange the constructed oil molecule model in an orderly manner into a cubic box; using the molecular filling algorithm, the oil molecules are filled into the box according to the principle of random but uniformity, while monitoring the distance between the molecules.

[0018] Preferably, the cross-sections of the oil molecule cubic box and the water molecule cubic box are adapted to the cross-sections of the first target cubic box.

[0019] Preferably, the volumes of the combined water box, oil box, and water box model are 1:2:1, and the oil-water ratio in the overall model is 0.6-0.8:1.

[0020] Preferably, the water-soluble chemical agent is any one of surfactants, polymers, and active nanoparticles, or a mixture thereof. The oil phase component model is simplified based on the target block; during the simulation, the temperature and pressure are kept consistent with the target block reservoir.

[0021] Preferably, the ratio of the oil phase active component to the inactive component is generally in the range of 1:2.5 to 3.1.

[0022] Preferably, the number of active molecules to be added to the water-soluble chemical added to the second target cubic box is calculated based on the concentration of the water-soluble chemical, and the amount of active molecules is obtained by n=m / M, and the number of molecules is then obtained by N=n*NA;

[0023] Where n is the amount of substance in moles (mol), m is the mass of the active molecule in grams (g), M is the molar mass of the active molecule in grams per mole (g / mol), and N is the number of active molecules (dimensionless quantity). A is Avogadro's constant, which is approximately 6.02×10 23 , which represents the number of particles contained in 1 mole of substance.

[0024] Preferably, the ratio of the oil phase active components to the oil phase inactive components in the crude oil and their respective molar masses are combined with the Avogadro constant to calculate the number of molecules of the oil phase active components and the oil phase inactive components, and the amount of substance of the oil phase active components and the oil phase inactive components is obtained by n=m / M, and then the number of molecules is obtained according to N=n*NA.

[0025] Preferably, the molecular dynamics equilibrium simulation is implemented using the Forcite program. The geometry optimization is specifically set as follows: using the Geometry Optimization task in the Forcite module, selecting the COMPASSⅡ force field, using Ewald summation for electrostatic force, using Atom-based summation for van der Waals force, and a step size of 2000.

[0026] The molecular dynamics equilibrium simulation is realized by the Forcite program, and the system is first subjected to NPT ensemble relaxation, and then to NVT ensemble relaxation.

[0027] The present invention proposes a molecular dynamics simulation method for the substitution adsorption characteristics of the oil-water interface by replacing the adsorption of oil-phase active molecules at the oil-water interface with water-soluble chemical molecules. After the water-soluble chemical molecule model is added to the first target cubic box, the system is subjected to molecular dynamics equilibrium simulation. The hydrophobic groups in the water-soluble chemical molecule model are inserted into the oil molecule cubic box due to hydrophobicity, while the hydrophilic groups are substituted and adsorbed at the oil-water interface.

[0028] In the first-purpose cubic box, a dynamic adsorption stable state is reached after molecular dynamics simulation. Since the hydrophilic groups in the oil phase active components, such as hydroxyl (-OH), carbonyl (-C=O) and other groups, and the oxygen atoms in the heteroatoms (N, S, etc.) in the molecules have a large electronegativity ratio and carry lone electron pairs, they will generate electrostatic attraction with the hydrogen atoms in the water molecules. At the same time, the hydrogen atoms on the hydrophilic groups form hydrogen bonds with the oxygen atoms in the water molecules.

[0029] Among them, water-soluble chemicals are added to the water phase of the second-purpose cubic box. After molecular dynamics equilibrium simulation, the hydrophilic groups therein will form hydrogen bonds with water molecules, and hydrophobic groups such as alkane side chains will move away from the water phase. After the water-soluble chemicals are adsorbed at the oil-water interface, they will change the molecular arrangement and interaction of the interface.

[0030] Since the hydrophilic groups interact with water, when the concentration of the water-soluble chemical agent is increased, the hydrophilic groups of the water-soluble chemical agent increase in unit volume, and the hydrophilic groups of the water-soluble chemical agent can occupy more adsorption sites to replace adsorption.

[0031] After the hydrophilic groups of water-soluble chemicals are adsorbed at the oil-water interface, steric hindrance will be generated due to the directional arrangement of molecules caused by the hydrophilic and hydrophobic groups.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (1) The present invention provides a molecular dynamics simulation method for the substitution adsorption characteristics of the oil-water interface. By adjusting the type and concentration of the water-soluble chemical agent and the temperature and pressure of the simulated environment, molecular dynamics simulation is performed to improve the substitution adsorption efficiency of the water-soluble chemical agent.

[0034] (2) The present invention analyzes the interaction between water-soluble chemicals and oil-phase active components and water, evaluates the interaction strength between the two and water at the microscopic level, and reveals the mechanism of substitution adsorption behavior; and supports this with radial distribution function and mean square displacement;

[0035] (3) The simulation results in the present invention are highly consistent with the preliminary experimental results, which verifies the accuracy of the prediction simulation and provides a theoretical basis for the application of surfactants in the oil and gas industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Surfactant molecular models;

[0037] Figure 2 It is a saturated molecular model;

[0038] Figure 3 is the asphaltene molecular model;

[0039] Figure 4 This is the initial state of the cubic box for the first purpose;

[0040] Figure 5 The initial state of the cubic box for the first purpose (hiding the inactive components of the oil phase);

[0041] Figure 6 The final state of the cubic box reaction for the first purpose;

[0042] Figure 7 This is the initial state of the second-purpose cubic box;

[0043] Figure 8 The initial state of the cubic box for the second purpose (hiding the inactive components of the oil phase);

[0044] Figure 9 The final state of the cubic box reaction for the second purpose;

[0045] Figure 10 Schematic diagram of surfactant at the oil-water interface;

[0046] Figure 11 RDF curves of asphaltene to water and oil at different surfactant concentrations;

[0047] Figure 12 is the mean square displacement curve of asphaltene molecules under different surfactant concentrations;

[0048] Figure 13 The interaction energy curves between asphaltene and water at different surfactant concentrations. DETAILED DESCRIPTION

[0049] The accompanying drawings are for illustrative purposes only. The technical contents of the present invention will be described in detail below in conjunction with the accompanying drawings and embodiments, but the technical contents of the present invention are not limited to the following embodiments.

[0050] Example 1

[0051] A molecular dynamics simulation method for substitution adsorption characteristics of an oil-water interface comprises the following steps:

[0052] 1. Construct oil phase active molecule model, oil phase inactive molecule model, water-soluble chemical agent molecule model and water molecule model:

[0053] Based on the characteristics and research needs of the target system, the construction work is started using the modeling module of professional molecular simulation software (such as Materials Studio). For water-soluble chemicals, the anionic surfactant sodium dodecyl sulfate (SDS) is selected. The long-chain alkyl and sulfate groups need to be accurately constructed to ensure that the connection method, bond length, and bond angle of each atom conform to chemical common sense; for the oil molecule model, asphaltene is selected as the active component and saturated components are selected as the inactive component; the water molecule model adopts the classic water molecule structure model, strictly ensuring that the bond length and bond angle of oxygen and hydrogen atoms are the standard values measured by experiments, so as to construct the basic molecular model unit, such as Figure 1 、 Figure 2 、 Figure 3 The ratio of active components in the oil phase to inactive components is 1:2.8, and the oil-water ratio in the overall model is 0.71:1.

[0054] 2. Construct the oil phase active molecule model and the oil phase inactive molecule model into an oil molecule cubic box:

[0055] Using the periodic boundary condition setting function in the simulation software, the constructed oil molecule model is arranged in an orderly manner in a cubic box. Using the molecular filling algorithm, the oil molecules are filled into the box in a random but uniform manner. At the same time, the distance between molecules is monitored to avoid excessive overlap or large spacing between molecules. This ensures that the oil molecules form a relatively dense and reasonably distributed aggregation state in the box, simulating the local accumulation state of the oil phase in the actual system. The box size is

[0056] Construct the water molecule model as a water molecule cubic box:

[0057] Similar to the process of constructing the oil molecule cubic box, the water molecule cubic box is set with an appropriate side length, taking into account the fluidity and hydrogen bonding properties of water. The water molecules are filled in using the same periodic boundary conditions and filling algorithm. However, during this process, the simulation parameters of the hydrogen bonding interaction between water molecules are specially activated so that the water molecules can automatically adjust their position and orientation according to the hydrogen bonding rules during the filling process, forming a hydrogen bonding network structure similar to that of real liquid water, showing the basic properties of the water phase. The box size is

[0058] 3. Combine the water molecule cube box and the oil molecule cube box into a water-oil-water combination model to obtain the first target cube box:

[0059] In the model editing interface of the simulation software, the constructed water molecule cubic box and the oil molecule cubic box are spliced in the order of water-oil-water. First, the relative positions of the two boxes are adjusted so that the oil molecule cubic box is precisely centered between the two water molecule cubic boxes, ensuring that the oil-water interface is flat and perpendicular to one of the coordinate axes of the box to facilitate subsequent analysis. Then, by integrating the boundary conditions, the unreasonable atomic overlap or gap at the splicing of the boxes is eliminated, making the entire combined model a continuous and seamless system, completing the construction of the first purpose cubic box and simulating the prototype of the hierarchical structure of the actual oil-water system. The size of the first purpose cubic box is like Figure 4 shown.

[0060] 4. Perform molecular dynamics equilibrium simulation on the first target cubic box to make the oil-water interface reach a stable state (dynamic equilibrium):

[0061] Select a suitable molecular dynamics simulation engine (such as the Forcite module) and set the simulation parameters for the first target cubic box. In terms of force field selection, consider the system components and select force fields such as the COMPASS force field that can accurately describe the interaction between oil, water and interfaces; electrostatic interactions are processed using the Ewald summation method to accurately calculate long-range electrostatic energy and ensure accurate charge interaction simulation; van der Waals interactions are set with a reasonable cutoff radius based on the atom type. To prevent omission of close-range interatomic forces, the simulation duration is set to 5-10ns, the step size is set to 1fs, and the thickness and interfacial tension of the oil-water interface are continuously monitored during the simulation. When the fluctuation of these parameters is less than the set threshold within a certain time range, it indicates that the oil-water interface has reached a stable dynamic equilibrium state. The size of the box after equilibrium is

[0062] 5. Observe and analyze the adsorption behavior of oil-phase active components at the oil-water interface:

[0063] like Figure 5 This is the initial state of the cubic box for the first purpose (hiding the inactive components of the oil phase). After the oil-water interface reaches a stable dynamic equilibrium state through molecular dynamics equilibrium simulation, the adsorption behavior of the active components of the oil phase at the oil-water interface is observed and analyzed. The purpose is to understand the natural adsorption of the active components of the oil phase itself at the oil-water interface when no surfactant is added, and to provide a basic reference for the subsequent study of the replacement adsorption behavior after the addition of surfactants, such as Figure 6 shown.

[0064] 6. Add the surfactant molecular model to the water boxes on both sides to obtain the second target cubic box, and relax the system again:

[0065] like Figure 7This is the initial state of the second purpose cubic box. Based on the first purpose cubic box that has reached equilibrium, the software's molecular insertion function is used to evenly add the pre-built surfactant molecule model to the water boxes on both sides at the set concentration. The diffusion characteristics of the surfactant should be considered during insertion to avoid excessive local concentration caused by the initial addition. After the addition is completed, the second purpose cubic box is obtained, and molecular dynamics relaxation simulation is performed on it again. The simulation parameters basically follow the settings of the previous equilibrium simulation, but the simulation time can be appropriately shortened to 4ns. Focus on the diffusion of surfactant molecules in the water phase, migration to the oil-water interface, and initial interaction with the oil-water interface. Ensure that the system regains a relatively stable state after the new component is added, and provide a reliable basis for subsequent research to replace adsorption behavior. Figure 8 This is the initial state of the cubic box for the second purpose (hiding the oil phase inactive components).

[0066] 7. Observe the adsorption behavior of surfactant-substituted asphaltene at the oil-water interface under equilibrium configuration and study the characteristics of surfactant-substituted adsorption behavior at the oil-water interface:

[0067] like Figure 9 This is the final state of the reaction of the second-purpose cubic box. In the visualization analysis module of the simulation software, a detailed observation of the second-purpose cubic box in equilibrium configuration is enabled. On the one hand, by marking different colors to track the movement trajectories of surfactant molecules and asphaltene molecules, their migration paths from the water phase or oil phase to the oil-water interface are viewed in real time, and the differences in migration rates are analyzed. On the other hand, using analytical tools such as radial distribution functions, the distribution probability of specific atoms around surfactant molecules and asphaltene molecules at the oil-water interface is statistically analyzed to infer their respective adsorption site preferences. For example, it is observed that water molecules are densely distributed around the hydrophilic groups of surfactants, and the degree of oil molecule aggregation around the aromatic ring structure of asphaltene, thus revealing the microscopic behavioral characteristics of the surfactant "replacing" the adsorption sites at the oil-water interface.

[0068] The characteristics of substitution adsorption behavior were studied by adjusting the type and concentration of surfactant as well as temperature and pressure:

[0069] (1) Type adjustment: Replace surfactants with different chemical structures, such as anionic to cationic or zwitterionic, and repeat the above steps 1-7. Compare and analyze the differences in molecular conformational changes, adsorption rate, adsorption stability, etc. of different types of surfactants when they are substituted and adsorbed at the oil-water interface, and explore the mechanism of the influence of chemical structure on substitution adsorption. Figure 10 Schematic diagram of surfactant at the oil-water interface.

[0070] (2) Concentration adjustment: Under the premise of keeping other conditions unchanged, gradually increase or decrease the initial concentration of the surfactant in the water phase and perform multiple simulations. Observe the coverage of the surfactant at the oil-water interface, the inhibitory effect on the asphaltene adsorption, and the change in the oil-water interfacial tension of the system as the concentration changes. Draw the curve of the relevant parameters changing with concentration, and quantitatively analyze the effect of concentration factors. Figure 11 are the RDF curves of asphaltene to water and oil at different surfactant concentrations. Figure 12 is the mean square displacement curve of asphaltene molecules under different surfactant concentrations, Figure 13 The interaction energy curves between asphaltene and water at different surfactant concentrations.

[0071] (3) Temperature and pressure adjustment: For temperature, increase or decrease the temperature of the simulation system in a certain step size (such as 10-20K), rerun the simulation, and monitor the impact of temperature changes on the thermal motion of surfactants and asphaltene molecules, the strength of intermolecular interactions, and the substitution adsorption equilibrium. For example, at high temperatures, molecular diffusion is accelerated but the adsorption stability may be reduced. For pressure, the software controls the pressure by changing the volume of the box, analyzes the effects of the oil-water interface area and molecular spacing changes under different pressures on the substitution adsorption characteristics of the two, and comprehensively reveals the regulation rules of environmental factors on substitution adsorption characteristics.

[0072] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the scope of protection of the present invention.

Claims

1. A molecular dynamics simulation method for substitution adsorption characteristics of oil-water interface, characterized in that: The steps include: Step 1: Construct the oil phase active molecule model, oil phase inactive molecule model, water-soluble chemical agent molecule model and water molecule model; Step 2: Construct the oil phase active molecule model and the oil phase inactive molecule model into an oil molecule cubic box, and construct the water molecule model into a water molecule cubic box; Step 3: Splice the water molecule cubic box and the oil molecule cubic box into a water-oil-water combination model with a sandwich layer structure to obtain the first target cubic box; Step 4: Perform molecular dynamics equilibrium simulation on the first target cubic box to make the oil-water interface reach a stable state, that is, the adsorption state of the oil-phase active components at the oil-water interface reaches dynamic equilibrium; Step 5: Observe and analyze the adsorption behavior of the oil-phase active components at the oil-water interface; Step 6: Randomly add the constructed water-soluble chemical molecular model to the water boxes on both sides to obtain the second target cubic box. Perform molecular dynamics simulation of the system again to observe the adsorption state of the water-soluble chemical at the oil-water interface under stable conditions and the changes in the adsorption state of the oil-phase active component at the oil-water interface. Step 7: Observe and analyze the adsorption behavior of water-soluble chemicals at the oil-water interface, and study the substitution adsorption behavior characteristics of water-soluble chemicals on the active components in the oil phase at the oil-water interface; The substitution adsorption behavior characteristics were studied by adjusting the type and concentration of water-soluble chemicals as well as temperature and pressure.

2. The molecular dynamics simulation method for oil-water interface substitution adsorption characteristics according to claim 1, characterized in that: The construction work was started using the modeling module of professional molecular simulation software. For the oil molecule model, asphaltene was selected as the active component and saturates were selected as the inactive component; the water molecule model adopted the classic water molecule structure model.

3. The molecular dynamics simulation method for oil-water interface substitution adsorption characteristics according to claim 2, characterized in that: Using the periodic boundary condition setting function in the simulation software, the constructed oil molecule model is arranged in an orderly manner into a cubic box; using the molecular filling algorithm, the oil molecules are filled into the box according to the principle of random but uniformity, while monitoring the distance between molecules.

4. The molecular dynamics simulation method for oil-water interface substitution adsorption characteristics according to claim 3, characterized in that: The cross sections of the oil molecule cubic box and the water molecule cubic box are adapted to the cross section of the first target cubic box.

5. The molecular dynamics simulation method for oil-water interface substitution adsorption characteristics according to claim 3, characterized in that: The volumes of the combined water box, oil box, and water box model are 1:2:1, and the oil-to-water ratio in the overall model is 0.6-0.8:

1.

6. The molecular dynamics simulation method for oil-water interface substitution adsorption characteristics according to claim 3, characterized in that: The water-soluble chemical agent is any one of a surfactant, a polymer and active nanoparticles or a mixture thereof.

7. The molecular dynamics simulation method for oil-water interface substitution adsorption characteristics according to claim 3, characterized in that: The ratio of active components to inactive components in the oil phase is generally in the range of 1:2.5 to 3.

1.

8. The molecular dynamics simulation method for oil-water interface substitution adsorption characteristics according to claim 3, characterized in that: The water-soluble chemical added to the second target cubic box is calculated based on the concentration of the water-soluble chemical to determine the number of active molecules. The amount of active molecules is obtained by n = m / M, and the number of molecules is obtained by N = n * NA. Where n is the amount of substance in mol, m is the mass of the active molecule in g, M is the molar mass of the active molecule in g / mol, and N is the number of active molecules. A is Avogadro's constant.

9. The molecular dynamics simulation method for oil-water interface substitution adsorption characteristics according to claim 8, characterized in that: The ratio of oil-phase active components to oil-phase inactive components in crude oil and their respective molar masses, combined with Avogadro's constant, are used to calculate the number of molecules of oil-phase active components and oil-phase inactive components. The amount of substance of oil-phase active components and oil-phase inactive components is obtained by n=m / M, and the number of molecules is then obtained according to N=n*NA.

10. The molecular dynamics simulation method for oil-water interface substitution adsorption characteristics according to claim 1, characterized in that: The molecular dynamics equilibrium simulation is implemented by the Forcite program, and the system is first subjected to NPT ensemble relaxation, and then to NVT ensemble relaxation.

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