Incorporating Nuclear Quantum Effects in Molecular Dynamics Simulations with Multicomponent Density Functional Theory

Nuclear quantum effects play an important role in a variety of chemical and biological processes and substantially affect many fundamental properties of systems involving hydrogen atoms. However, the accurate inclusion of nuclear quantum effects remains a significant challenge for large-scale molecular simulations. We present an alternative formulation of equations of motion for molecular dynamics (MD) based on a constrained minimized energy surface (CMES). Since CMES inherently includes nuclear quantum effects, the resulting CMES-MD is able to capture nuclear quantum effects such as quantum delocalization effects and tunneling effects. In model systems, CMES-MD gives results that are comparable to or better than those from centroid molecular dynamics and ring-polymer molecular dynamics. In practical molecular systems, CMES can be obtained from constrained nuclear-electronic orbital density functional theory (cNEO-DFT), a multicomponent density functional theory that was recently developed in our group. The resulting cNEO-MD is employed to calculate the vibrational spectra of a series of small molecules and the results are compared to those from conventional ab initio molecular dynamics (AIMD) as well as from experiments. With the same formal computational scaling, cNEO-MD greatly outperforms AIMD in describing the vibrational modes with a significant hydrogen motion character, indicating the importance of nuclear quantum effects in molecular simulations. This work opens the door to the accurate and efficient simulation of chemical and biological systems with significant nuclear quantum effects using cNEO-MD.