The strengths and limitations of effective centroid force models explored by studying isotopic effects in liquid water

Abstract

The development of effective centroid potentials (ECPs) is explored with both the constrained-centroid and quasi-adiabatic force matching using liquid water as a test system. A trajectory integrated with the ECP is free of statistical noises that would be introduced when the centroid potential is approximated on the fly with a finite number of beads. With the reduced cost of ECP, challenging experimental properties can be studied in the spirit of centroid molecular dynamics. The experimental number density of HO is 0.38% higher than that of DO. With the ECP, the HO number density is predicted to be 0.42% higher, when the dispersion term is not refit. After correction of finite size effects, the diffusion constant of HO is found to be 21% higher than that of DO, which is in good agreement with the 29.9% higher diffusivity for HO observed experimentally. Although the ECP is also able to capture the redshifts of both the OH and OD stretching modes in liquid water, there are a number of properties that a classical simulation with the ECP will not be able to recover. For example, the heat capacities of HO and DO are predicted to be almost identical and higher than the experimental values. Such a failure is simply a result of not properly treating quantized vibrational energy levels when the trajectory is propagated with classical mechanics. Several limitations of the ECP based approach without bead population reconstruction are discussed.

Simulating a flexible water model as rigid: Best practices and lessons learned

“…Performing path integral calculations to determine the T M is very challenging. Presumably, the effective centroid potential 49 can be used to obtain a converged estimate. However, even such a simulation would still be quite challenging and is beyond the scope of this work.…”

Performing Molecular Dynamics Simulations and Computing Hydration Free Energies on the B3LYP-D3(BJ) Potential Energy Surface with Adaptive Force Matching: A Benchmark Study with Seven Alcohols and One Amine

“…Also, some properties may require explicit considerations of nuclear quantum effects. 101 While we are confident that there are other properties where AFM based models would provide accurate predictions, the actual performance for properties not studied in this work would need to be further investigated and validated.…”

“…For some properties, the simplistic energy expressions used would cause important physics to be missed, and thus the prediction may not be accurate. Also, some properties may require explicit considerations of nuclear quantum effects . While we are confident that there are other properties where AFM based models would provide accurate predictions, the actual performance for properties not studied in this work would need to be further investigated and validated.…”

Path-integral approximations to quantum dynamics

“…We have focused mostly on vibrational spectra, but should point out that many of the above techniques can be used to improve the efficiency of RPMD calculations of thermal reaction rates [10,29,30,38,39,[82][83][84][85][86][87][88], and diffusion constants [31,32,[116][117][118][119][120][121][122] (for which which both CMD and TRPMD appear to work well, probably because any diffusive dynamics that satisfies quantum detailed balance and has the centroid as the diffusion coordinate is sufficient). Particularly noteworthy pathintegral simulations of diffusion constants include simu-Fig.…”