title: Combining Internally Contracted States and Matrix Product States To Perform Multireference Perturbation Theory.
abstract: We present two efficient and intruder-free methods for treating dynamic correlation on top of general multiconfiguration reference wave functions - including such as obtained by the density matrix renormalization group (DMRG) with large active spaces. The new methods are the second order variant of the recently proposed multireference linearized coupled cluster method (MRLCC) [ Sharma, S.; Alavi, A. J. Chem. Phys. 2015 , 143 , 102815 ] and of N-electron valence perturbation theory (NEVPT2), with expected accuracies similar to MRCI+Q and (at least) CASPT2, respectively. Great efficiency gains are realized by representing the first order wave function with a combination of internal contraction (IC) and matrix product state perturbation theory (MPSPT). With this combination, only third order reduced density matrices (RDMs) are required. Thus, we obviate the need for calculating (or estimating) RDMs of fourth or higher order; these had so far posed a severe bottleneck for dynamic correlation treatments involving the large active spaces accessible to DMRG. Using several benchmark systems, including first and second row containing small molecules, Cr2, pentacene, and oxo-Mn(Salen), we show that active spaces containing at least 30 orbitals can be treated using this method. On a single node, MRLCC2 and NEVPT2 calculations can be performed with over 550 and 1100 virtual orbitals, respectively. We also critically examine the errors incurred due to the three sources of errors introduced in the present implementation - calculating second order instead of third order energy corrections, use of internal contraction, and approximations made in the reference wave function due to DMRG.
authors.affiliation: Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany.|Department of Chemistry and Biochemistry, University of Colorado Boulder , Boulder, Colorado 80302, United States.
authors.forename: Sandeep
authors.identifier: http://orcid.org/0000-0002-6598-8887
authors.initials: S
authors.lastname: Sharma
authors.affiliation: Department of Chemistry, Pennsylvania State University , 401A Chemistry Building, University Park, Pennsylvania 16802, United States.
authors.forename: Gerald
authors.initials: G
authors.lastname: Knizia
authors.affiliation: Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States.
authors.forename: Sheng
authors.initials: S
authors.lastname: Guo
authors.affiliation: Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany.|Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Ali
authors.initials: A
authors.lastname: Alavi