Use of π-Space CASSCF Trial Wave Functions in DMC Calculations of Unsaturated Organic Molecules

The major source of error in fixed-node diffusion Monte Carlo (DMC) electronic structure calculations is the inadequate description of the nodal surface for electron exchange. In this work, we examine the use of π-space complete active space self-consistent field (CASSCF) trial wave functions for several unsaturated hydrocarbons and their derivatives. We find that reoptimizing the CI coefficients of the CASSCF wave function in the VMC step together with parameters in the Jastrow factor leads to substantial lowering of the DMC energies. In general, this reoptimization causes significant reduction in magnitude of the coefficients of the minor configurations in wave functions. Moreover, it is shown for five of the systems studied that one can obtain a similar energy lowering by simply scaling all secondary configurations by a factor of ∼0.5. It is further demonstrated that one can predict, often to within chemical accuracy, the energy lowering when going from single-determinant DMC to CAS-DMC based on the magnitude of the dominant coefficient of the trial wave function, both across different molecules and along structural distortions.