Molecular Electrical Properties from Quantum Monte Carlo Calculations: Application to Ethyne.

We used Quantum Monte Carlo (QMC) methods to study the polarizability and the quadrupole moment of the ethyne molecule using the Jastrow-Antisymmetrised Geminal Power (JAGP) wave function, a compact and strongly correlated variational ansatz. The compactness of the functional form and the full optimization of all its variational parameters, including linear and exponential coefficients in atomic orbitals, allow us to observe a fast convergence of the electrical properties with the size of the atomic and Jastrow basis sets. Both variational results on isotropic polarizability and quadrupole moment based on Gaussian type and Slater type basis sets are very close to the Lattice Regularized Diffusion Monte Carlo values and in very good agreement with experimental data and with other quantum chemistry calculations. We also study the electronic density along the C≡C and C-H bonds by introducing a generalization for molecular systems of the small-variance improved estimator of the electronic density proposed by Assaraf et al. (Assaraf, R.; Caffarel, M.; Scemama, A. Phys. Rev. E, 2007, 75, 035701).