On the systematic improvement of fixed-node diffusion quantum Monte Carlo energies using pair natural orbital CI guide functions

While the diffusion quantum Monte Carlo method (DQMC) is capable, in princi ple, of calculating exact ground state energies, in practice the fixed-node (FN) approximation leads to node location errors which make FN-DQMC energi es upper bounds. It is shown that the node location error can be reduced sy stematically and without prohibitive increase of computer time requirements by using nodes derived from pair natural orbital CI wave functions (PNO-CI ). The reduction is demonstrated for the N atom and the molecules N-2 and H 2O. With the DQMC/PNOCI method, we obtain a variational energy of -109.520( 3) H for the N-2 molecule and -76.429(1) H for the ground state of the wate r molecule which is only 22 and 9 mH above the estimated nonrelativistic gr ound state energy, respectively. (C) 2000 American Institute of Physics. [S 0021-9606(00)30943-6].