A soft Hartree-Fock pseudopotential for carbon with application to quantum Monte Carlo

We describe the development of an ab initio Hartree-Fock (HF) pseudopotential for C that has neither repulsive r(-2) nor attractive Coulomb singularities at the nucleus. This potential is tested: in HF calculations on small hydrocarbons and shown to give good accuracy in comparison to all-electron calculations. The new pseudopotential is demonstrated to give superior basis;set convergence in HF calculations compared to standard ones with r(-2) singularities. Diffusion Monte Carlo (DMC) calculations on C and CH with the new potential show substantial improvements in stability at larger time steps compared with conventional pseudopotentials, with negligible time Step dependence of the total energy. The resultant CH bond energy, D-e = 83.3(4) kcal/mol, is in good agreement with experiment and with a similar all-electron DMC calculation. (C) 1998 American Institute of Physics.