The Adaptive Shift Method in Full Configuration Interaction Quantum Monte Carlo: Development and Applications

In a recent paper, we proposed the adaptive shift method for correcting the undersampling bias of the initiator-FCIQMC. The method allows faster convergence with the number of walkers to the FCI limit than the normal initiator method, particularly for large systems. In its application to strongly correlated molecules, however, the method is prone to overshooting the FCI energy at intermediate walker numbers, with convergence to the FCI limit from below. In this paper, we present a solution to the overshooting problem in strongly correlated molecules, as well as further accelerating convergence to the FCI energy. This is achieved by offsetting the reference energy to a value typically below the Hartree-Fock energy but above the exact energy. This offsetting procedure does not change the exactness property of the algorithm, namely convergence to the exact FCI solution in the large-walker limit, but at its optimal value greatly accelerates convergence. There is no overhead cost associated with this offsetting procedure, and is therefore a pure and substantial computational gain. We illustrate the behavior of this offset adaptive shift method by applying it to the N₂ molecule, the ozone molecule at three different geometries (equilibrium open minimum, a hypothetical ring minimum, and a transition state) in three basis sets cc-pV$X$Z, $X$=D,T,Q, and the chromium dimer in cc-pVDZ basis set, correlating 28 electrons in 76 orbitals. We show that in most cases the offset adaptive shift method converges much faster than both the normal initiator method and the original adaptive shift method.