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CASSCF with Extremely Large Active Spaces Using the Adaptive Sampling Configuration Interaction Method
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DOI：10.1021/acs.jctc.9b01255.png)
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Abstract
En 中文
The complete active space self-consistent field (CASSCF) method is the principal approach employed for studying strongly correlated systems. However, exact CASSCF can only be performed on small active spaces of similar to 20 electrons in similar to 20 orbitals due to exponential growth in the computational cost. We show that employing the Adaptive Sampling Configuration Interaction (ASCI) method as an approximate Full CI solver in the active space allows CASSCF-like calculations within chemical accuracy (<1 kcal/mol for relative energies) in active spaces with more than similar to 50 active electrons in similar to 50 active orbitals, significantly increasing the sizes of systems amenable to accurate multi-configurational treatment. The main challenge with using any selected CI-based approximate CASSCF is the orbital optimization problem; they tend to exhibit large numbers of local minima in orbital space due to their lack of invariance to active-active rotations (in addition to the local minima that exist in exact CASSCF). We highlight methods that can avoid spurious local extrema as a practical solution to the orbital optimization problem. We employ ASCI-SCF to demonstrate a lack of polyradical character in moderately sized periacenes with up to 52 correlated electrons and compare against heat-bath CI on an iron porphyrin system with more than 40 correlated electrons.
Keywords:
DENSITY-FUNCTIONAL THEORY
SELF-CONSISTENT-FIELD
2ND-ORDER PERTURBATION-THEORY
ELECTRONIC GROUND-STATE
ORBITAL OPTIMIZATION
COUPLED-CLUSTER
EXCITED-STATES
HARTREE-FOCK
BASIS-SETS
ENERGIES
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