Intermolecular forces and fixed-node diffusion Monte Carlo: A brute force test of accuracies for He2 and He–LiH

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Abstract
The accuracy of the fixed-node approximation and diffusion Monte Carlo method in computing the interaction energy of van der Waals systems was investigated. Tests were carried out by simulating the electronic structures of He2 and He–LiH. These two systems were chosen as representative of two fundamentally different interactions, namely the weak dispersion forces in He2 and the dipole/induced–dipole interaction in He–LiH. The results for both systems are in excellent agreement with “state of the art” calculations, thereby indicating a high accuracy for the fixed-node approximation. Also, our interaction energies for He–LiH indicate that the coupled cluster single double triple method [Taylor and Hinde, J. Chem. Phys. 111, 973 (1999)] gives an accurate prediction of the interaction potential for that system.