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SUMMARY:Multireference Stochastic Coupled Cluster - Maria-Andreea Filip\, 
 University of Cambridge
DTSTART:20191113T141500Z
DTEND:20191113T143500Z
UID:TALK130447@talks.cam.ac.uk
CONTACT:Lisa Masters
DESCRIPTION:Stochastic electronic structure methods\, such as Full Configu
 ration Interaction Quantum Monte Carlo and Coupled Cluster Monte Carlo\, h
 ave become popular in recent years as they significantly improve computati
 onal and memory costs compared to their determininstic counterparts. Howev
 er\, they suffer from much the same drawbacks\, such as difficulties in tr
 eating systems with multireference character by coupled cluster. We presen
 t a novel multireference modification to the stochastic coupled cluster al
 gorithm\, based on a single reference formalism. By treating secondary ref
 erences as excitations of the primary reference\, while also allowing them
  to change the selection and spawning criteria in the Monte Carlo propagat
 ion\, we obtain a method that preserves the appealing simplicity of the si
 ngle-reference algorithm\, while significantly improving performance for k
 nown strongly correlated systems\, such as the stretched nitrogen molecule
 .4 For this system\, a calculation using two references successfully captu
 res the correlation energy (to within chemical accuracy) at the CCSDT leve
 l. The method can be trivially expanded to an arbitrary numbers of referen
 ces\, giving results that compare favourably to previous complete active s
 pace (CAS) based multireference coupled cluster methods. The ability to ca
 refully control the included references\, as well as their allowed excitat
 ion levels\, provides us with a new tool to better understand the importan
 ce of CAS determinants to the quality of the predicted properties.
LOCATION:Department of Chemistry\, Cambridge\, Unilever lecture theatre
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