Quantum Monte Carlo study of the dipole moment of CO

We have calculated the dipole moment of CO along the potential energy curve using the pure diffusion quantum Monte Carlo method. Good agreement with coupled cluster singles-doubles calculations has been obtained throughout. An accurate treatment at the equilibrium distance requires a multi-reference trial wave function for which we obtained a dipole moment in accordance with experiment. We discuss the Hellmann-Feynman theorem within the fixed-node approximation in the case of field-dependent smoothly varying nodes. It is shown that the Hellmann-Feynman theorem is applicable to this case due to vanishing boundary contributions. A comparison has been made between the finite field correlated sampling approach and a direct calculation of the expectation value for the dipole moment operator. In the present application both methods perform equally well. (C) 1999 American Institute of Physics. [S0021-9606(99)30524-9].