Alavi 2017

Ali Alavi (Max Planck Institute for Solid State Research)
Relaxing the initiator approximation of i-FCIQMC
15 November 2017 (Wed) 15:00-16:00 S514 Seminar Room
We describe a method to relax the initiator approximation at constant number of walkers. The method consists of the identification of sign-coherent determinants, which are allowed to spawn freely, even if their population is below the initiator threshold. The result is a dramatic improvement in the efficiency of  the i-FCIQMC method, allowing challenging systems to be tackled with relatively low walker numbers. For example, with the new scheme, butadiene can be converged with 20 million walkers, whereas with the normal initiator even 4 billion walkers does not suffice. We use the method to provide a FCI estimate of the benzene molecule, fully correlating all 30 valence electrons in 108 orbitals. The results are compared to high order CC methods going upto CCSDT(Q). If time allows, I will also describe a recent extension of the FCIQMC method to solving transcorrelated Hamiltonians, providing a new method to reach the basis set limit in multi-configurational problems.