Dipole moments of ultra-cold polar molecules: a quantum Monte Carlo study

Recently, there has been a great deal of interest in the production of ultra-cold heteronuclear molecules having large electric dipole moments [1]. This is of interest both for fundamental reasons as well as for applications such as qubits for quantum computing [2]. In this work, we calculate the dipole moment of a potentially implementable two-atom alkaline-alkaline-earth molecule, LiSr. We use correlated wave-function methods including both the quantum chemical configuration interaction (CI) method, and a stochastic quantum Monte Carlo (QMC), to calculate the potential energy surface and dipole moment. We study the dipole moment with increasing accuracy of correlated wave-functions. We then variationally re-optimize the wave-functions, which then serve as the representation of the Fermion nodes in the fixed-node QMC. To treat the Sr atom we employ two types of effective core potentials (ECPs), large core ECPs have only $s$-states in the valence space, while the small core ECP's valence space includes also the highest $s$ and $p$ semi-core sub-shells. We find significant sensitivity of thedipole moment on both the size of the valence space and on the accuracy of the Fermion nodes. [1] B. Damski, {\it et. al} Phys. Rev. Lett. 90, 110401 (2003). [2] D. DeMille, Phys. Rev. Lett. 88, 067901 (2002).