The hole mass in Car-Parrinello molecular dynamics: insights into the dynamics of excitation

In the Car-Parrinello molecular dynamics (CPMD) formalism, orbitals can be assigned different effective masses according to whether the orbital is occupied by a hole or an electron, and such masses affect the response of the orbitals to their environment. Inspired by this, we introduce and implement a novel modification of CPMD, HoleMass CPMD, in which a hole, which is a partially empty orbital, is assigned a fictitious mass that is different from fully occupied orbitals. Despite the simplicity of the approach, we find that it solves a key problem in first principles molecule dynamics simulation: for a set of carefully assigned mass values, the method is able to successfully simulate photoinduced chemical reactions, exemplified here by the ring-opening reaction in oxirane within a few femtoseconds, and cyclobutene, within a few picoseconds. Our method can reproduce the CO ring-opening of oxirane, and the correct isomerization sequence for cyclobutene: when the ring opens, the first isomer that forms is the cis isomer, followed by the trans isomer. Our method has been implemented in the Car-Parrinello package of QuantumEspresso and is available as an open-source contribution. The HoleMass CPMD method provides a new quantum chemistry tool for the simulation of excitation dynamics in molecules, and can also be applied for modelling charge localization effects in materials systems.