Quantum computing study of molecular properties treating nuclear quantum effects

Abstract

Molecular orbital (MO) simulation enables us to tackle various chemical problems. However, there are some cases where conventional MO simulation cannot describe the chemical problem accurately, such as isotope effects. Treatment of nuclear quantum effects are essential to tackle these problems. The Multicomponent Molecular Orbital (MCMO) method treats the nuclei quantum mechanically and allows us to include the nuclear quantum effects in the quantum chemistry simulations, but requires larger computational resources compared to conventional MO simulations. Therefore, MCMO simulations are limited to smaller-size molecular systems compared to conventional MO simulations. The use of quantum computers for MCMO simulations is appealing. Some studies have investigated MCMO simulation using quantum algorithms, however, they are limited in number. In this work, we investigate calculation of molecular properties using MCMO simulation with quantum algorithms including simulations on quantum devices. We computed the dipole moments of HD molecule, which cannot be calculated using conventional MO methods. Variational quantum eigensolver with UCCSD ansatz was used for computing the dipole moments. We further combined the MCMO method with Frozen Natural Orbital approach for virtual space truncation to reduce the problem size for the use of near-term quantum devices. We studied the effect of removing translational and rotational motions of the system on the calculation of the dipole moments. We confirmed that removing the rotational motions of the system is essential for accurate computation of the dipole moments of HD.