Adaptive Force Matching Method Research Articles

Water graphene contact surface investigated by pairwise potentials from force-matching PAW-PBE with dispersion correction.

A pairwise additive atomistic potential was developed for modeling liquid water on graphene. The graphene-water interaction terms were fit to map the PAW-PBE-D3 potential energy surface using the adaptive force matching method. Through condensed phase force matching, the potential developed implicitly considers the many-body effects of water. With this potential, the graphene-water contact angle was determined to be 86° in good agreement with a recent experimental measurement of 85° ± 5° on fully suspended graphene. Furthermore, the PAW-PBE-D3 based model was used to study contact line hysteresis. It was found that the advancing and receding contact angles of water do agree on pristine graphene, however a long simulation time was required to reach the equilibrium contact angle. For water on suspended graphene, sharp peaks in the water density profile disappear when the flexibility of graphene was explicitly considered. The water droplet induces graphene to wrap around it leading to a slightly concave contact interface.

Achieving fast convergence of ab initio free energy perturbation calculations with the adaptive force-matching method

This paper studies the possibility of improving the convergence of ab initio free energy perturbation (FEP) calculations by developing customized force fields with the adaptive force-matching (AFM) method. The ab initio FEP method relies on a molecular mechanics (MM) potential to sample configuration space. If the Boltzmann weight of the MM sampling is close to that of the ab initio method, the efficiency of ab initio FEP will be optimal. The difference in the Boltzmann weights can be quantified by the relative energy difference distribution (REDD). The force field developed through AFM significantly improves the REDD when compared with standard MM models, thus improving the convergence of the ab initio FEP calculation. The static dielectric constant es of ice-Ih was studied with PW-91 through ab initio FEP. With a customized force field developed through AFM, we were able to converge es to 80 ± 4 with 3,600 configurations. A similar ab initio FEP calculation with the TIP4P model would require 220 times more configurations to achieve the same accuracy. Our study indicates that the PW-91 functional underestimates ice-Ih es by about 20%.

Approaching post-Hartree–Fock quality potential energy surfaces with simple pair-wise expressions: parameterising point-charge-based force fields for liquid water using the adaptive force matching method

This article summarises the adaptive force matching (AFM) method recently developed by the Wang group. The AFM method is capable of parameterising force fields in the condensed phase by fitting electronic structure forces obtained through quantum mechanics/molecular mechanics (QM/MM) calculations. The AFM method utilises an iterative procedure to ensure good sampling and high-quality MM models for QM/MM calculations. By fitting forces directly in the condensed phase, simple pair-wise additive potentials parameterised following AFM can account for many-body effects implicitly. The AFM method provides a mechanism for judging the effectiveness of force field terms and the quality and transferability of the final force field. Realising the limitation of simple potentials, AFM emphasises fitting problem-specific force fields instead of a universal force field. This article provides a demonstration using the recently developed density functional theory/supplemental potential method to provide reference forces to parameterise a four-site point-charge model, BLYPSP-4F, for liquid water in the temperature range from 0 to 40°C. No experimental information was used for the development of BLYPSP-4F the model is found to satisfactorily reproduce experimental properties.

A simple molecular mechanics potential for μm scale graphene simulations from the adaptive force matching method

A simple molecular mechanics force field for graphene (PPBE-G) was created by force matching the density functional theory Perdew-Burke-Ernzerhof forces using the adaptive force matching method recently developed in our group. The PPBE-G potential was found to provide significantly more accurate forces than other existing force fields. Several properties of graphene, such as Young's modulus, bending rigidity, and thermal conductivity, have been studied with our potential. The calculated properties are in good agreement with corresponding density functional theory and experimental values. The thermal conductivity calculated with reverse non-equilibrium molecular dynamics depends sensitively on graphene size thus requiring the simulation of large sheets for convergence. Since the PPBE-G potential only contains simple additive energy expressions, it is very computationally efficient and is capable of modeling large graphene sheets in the μm length scale.

The quest for the best nonpolarizable water model from the adaptive force matching method

The recently introduced adaptive force matching (AFM) method is used to develop a significantly improved pair-wise nonpolarizable potential for water. A rigid version of the potential is also presented to enable larger time steps for biological simulations. In this work, it is demonstrated that the AFM method can be used to systematically assess the importance of each functional term during the construction of a force field. For a water potential, it is established that a single off-atom charge center (M) in the plane of water outperforms two out-of-plane charge sites for reproducing intermolecular forces. The four-site pair-wise nonpolarizable force field developed in this work rivals some of the most sophisticated polarizable models in terms of reproducing accurate ab initio forces. The force fields are parameterized to perform best in the temperature range from 0 to 40°C. Equilibrium and dynamical properties calculated with the flexible and rigid force fields are in good agreement with experimental results. For the flexible model, the agreement improves when path integral simulation is performed. These force fields provide high-quality results at a very low computational cost and are thus well suited to atomistic scale biological simulations. The AFM method provides a mechanism for selecting important terms in force field expressions and is a very promising tool for producing accurate force fields in condensed phases.

Improving the Point-Charge Description of Hydrogen Bonds by Adaptive Force Matching

The MP2f water force field, recently developed using the adaptive force matching method, is improved significantly after the introduction of a short-range repulsion term. This term is designed to address the deficiency of point-charge models at short charge-charge separations. It may also provide a more realistic description of hydrogen bonds by capturing exchange-repulsion interactions. When compared with MP2f, the new MP2f_hb force field predicts properties of liquid water in much better agreement with experiments. The calculated site-site radial distribution functions are in good agreement with X-ray diffraction data corroborating the argument that point-charge models with only Lennard-Jones repulsion underestimate the nearest neighbor O-O distance. The incorporation of the new short-range repulsion term is argued to be important for any force field model that uses point charges to model electrostatics.

Developing ab initio quality force fields from condensed phase quantum-mechanics/molecular-mechanics calculations through the adaptive force matching method

A new method called adaptive force matching (AFM) has been developed that is capable of producing high quality force fields for condensed phase simulations. This procedure involves the parametrization of force fields to reproduce ab initio forces obtained from condensed phase quantum-mechanics/molecular-mechanics (QM/MM) calculations. During the procedure, the MM part of the QM/MM is iteratively improved so as to approach ab initio quality. In this work, the AFM method has been tested to parametrize force fields for liquid water so that the resulting force fields reproduce forces calculated using the ab initio MP2 and the Kohn-Sham density functional theory with the Becke-Lee-Yang-Parr (BLYP) and Becke three-parameter LYP (B3LYP) exchange correlation functionals. The AFM force fields generated in this work are very simple to evaluate and are supported by most molecular dynamics (MD) codes. At the same time, the quality of the forces predicted by the AFM force fields rivals that of very expensive ab initio calculations and are found to successfully reproduce many experimental properties. The site-site radial distribution functions (RDFs) obtained from MD simulations using the force field generated from the BLYP functional through AFM compare favorably with the previously published RDFs from Car-Parrinello MD simulations with the same functional. Technical aspects of AFM such as the optimal QM cluster size, optimal basis set, and optimal QM method to be used with the AFM procedure are discussed in this paper.