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SUMMARY:Approaching Exact Quantum Chemistry by Stochastic Wave Function Sa
 mpling and Deterministic Coupled-Cluster Computations  - Professor Piotr P
 iecuch\, Michigan State University
DTSTART:20200304T141500Z
DTEND:20200304T151500Z
UID:TALK127531@talks.cam.ac.uk
CONTACT:Lisa Masters
DESCRIPTION:One of the main goals of electronic structure theory is to pre
 cisely describe increasingly complex polyatomic systems. It is widely acce
 pted that size extensive methods based on the exponential wave function an
 satz of coupled-cluster (CC) theory and their extensions to excited states
  via the equation-of-motion (EOM) formalism are excellent candidates for a
 ddressing this goal. In this talk\, I will discuss a radically new way of 
 obtaining accurate energetics equivalent to high-level CC calculations\, s
 uch as CCSDT or CCSDTQ\, at a small fraction of the computational cost\, e
 ven when multireference correlation effects become significant\, resulting
  from the merger of a deterministic formalism\, abbreviated as CC(P\;Q) [1
 \,2]\, with the stochastic CI [3\,4] and CC [5] Quantum Monte Carlo (QMC) 
 approaches [6]. I will also demonstrate that one can take the merger of st
 ochastic and deterministic ideas to the ultimate level and use it to extra
 ct the exact\, full CI (FCI)\, energetics out of the early stages of FCIQM
 C propagations with the help of the relatively inexpensive polynomial step
 s similar to CCSD calculations\, eliminating exponential complexity of con
 ventional FCI Hamiltonian diagonalizations altogether [7]. The advantages 
 of the new methodologies will be illustrated by molecular examples\, where
  the goal is to recover the nearly exact\, CCSDT and CCSDTQ\, and exact\, 
 FCI\, energetics in situations involving chemical bond dissociations and r
 eaction pathways and many-electron systems beyond the reach of FCI. Extens
 ions to excited electronic states by a combination of stochastic CIQMC and
  deterministic EOMCC computations [8] and converging FCI energetics in str
 ongly correlated systems\, such as those involved in modeling metal–insu
 lator transitions [9]\, where the traditional CCSD\, CCSDT\, CCSDTQ\, etc.
  hierarchy breaks down\, will be discussed as well.\n\nReferences\n[1]	J. 
 Shen and P. Piecuch\, Chem. Phys. 401\, 180 (2012)\; J. Chem. Phys. 136\, 
 144104 (2012).\n[2]	N. P. Bauman\, J. Shen\, and P. Piecuch\, Mol. Phys. 1
 15\, 2860 (2017).\n[3]	G. H. Booth\, A. J. W. Thom\, and A. Alavi\, J. Che
 m. Phys. 131\, 054106 (2009).\n[4]	D. Cleland\, G. H. Booth\, and A. Alavi
 \, J. Chem. Phys. 132\, 041103 (2010).\n[5]	A. J. W. Thom\, Phys. Rev. Let
 t. 105\, 263004 (2010).\n[6]	J. E. Deustua\, J. Shen\, and P. Piecuch\, Ph
 ys. Rev. Lett. 119\, 223003 (2017)\; in preparation.\n[7]	J. E. Deustua\, 
 I. Magoulas\, J. Shen\, and P. Piecuch\, J. Chem. Phys. 149\, 151101 (2018
 ).\n[8]	J. E. Deustua\, S. H. Yuwono\, J. Shen\, and P. Piecuch\, J. Chem.
  Phys. 150\, 111101 (2019)\; S. H. Yuwono\, A. Chakraborty\, J. E. Deustua
 \, J. Shen\, and P. Piecuch\, in preparation.\n[9]	I. Magoulas\, J. E. Deu
 stua\, J. Shen\, and P. Piecuch\, in preparation.\n
LOCATION:Department of Chemistry\, Cambridge\, Unilever lecture theatre
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