Journal
JOURNAL OF CHEMICAL PHYSICS
Volume 145, Issue 5, Pages -Publisher
AIP Publishing
DOI: 10.1063/1.4959817
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Funding
- Research Foundation Flanders (Fonds Wetenschappelijk Onderzoek Vlaanderen)
- Ghent University
- Hercules Foundation
- Flemish Government-Department EWI
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We have implemented internally contracted complete active space second order perturbation theory ( CASPT2) with the density matrix renormalization group ( DMRG) as active space solver [ Y. Kurashige and T. Yanai, J. Chem. Phys. 135, 094104 ( 2011)]. Internally contracted CASPT2 requires to contract the generalized Fock matrix with the 4-particle reduced density matrix ( 4-RDM) of the reference wavefunction. The required 4-RDM elements can be obtained from 3-particle reduced density matrices ( 3-RDM) of different wavefunctions, formed by symmetry-conserving single-particle excitations op top of the reference wavefunction. In our spin-adapted DMRG code CHEMPS2 https://github.com/sebwouters/chemps2, we decompose these excited wavefunctions as spin-adapted matrix product states and calculate their 3-RDM in order to obtain the required contraction of the generalized Fock matrix with the 4-RDM of the reference wavefunction. In this work, we study the longitudinal static second hyperpolarizability of all-trans polyenes C2nH2n+2 [ n = 4-12] in the cc-pVDZ basis set. DMRG-SCF and DMRG-CASPT2 yield substantially lower values and scaling with system size compared to RHF and MP2, respectively. Published by AIP Publishing.
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