Potential Energy Surfaces of Hydrogen-bond and Halogen-bond Systems with Diffuse Quantum Monte Carlo Method

Fixed-node diffusion Monte Carlo (FN-DMC) method is known to provide reliable interaction energies for hydrogen-bond and halogen-bond systems using a single-determinant-Jastrow (SDJ) trial wavefunction. However, previous FN-DMC calculations were only carried out at equilibrium distances. In this work, performance of FN-DMC on interaction energies of hydrogen-bond and halogen-bond systems at non-equilibrium distances is investigated by calculating the potential energy surfaces of 16 such systems. Effects of basis sets, Jastrow factors, two schemes to treat the non-local part of pseudopotentials (PPs), namely the T-move approximation and the determinant localization approximation (DLA), on FN-DMC results for these systems are also studied. According to our results, FN-DMC can provide reliable interaction energies at non-equilibrium distances for these systems. The basis set has a slightly larger effect on the total energy using T-move than that using DLA, while FN-DMC interaction energies with T-move are slightly less sensitive to the choice of basis set than those with DLA. Effects of basis sets, treatment of PPs and Jastrow factor on interaction energies with FN-DMC are insignificant. In addition, using the Jastrow factors optimized at equilibrium distances for non-equilibrium bond length or neglecting the three-body term in the Jastrow factor in FN-DMC calculations can also provide reasonable results even with T-move. In terms of computational cost, the aug-cc-pVTZ basis set combined with DLA is a feasible choice in FN-DMC calculations for larger hydrogen-bond and halogen-bond systems.