
Relativistic manybody methods 

Relativistic GAS Coupled Cluster of General Excitation Rank

For the most efficient treatment of dynamic electron correlation, a relativistic quasiMRCC
approach of general excitation rank has been developed. The nonrelativistic precursor
program was
introduced by Olsen (J. Chem. Phys. 113,17 (2000) 71407148) which uses higher
excitations defined by the Generalized Active Space (GAS) concept to simulate
multireference model spaces (see image).
For electronic ground states

both a determinantbased (CI expansion based) version (see ref. Theo. Chem. Acc. 2007)
and a commutatorbased version (see ref. J. Chem. Phys. 2011),
the latter with a correct computational scaling as conventional CC approaches (see Z Phys Chem 2010)
have been devised. These implementations assume a basis of Kramerspaired spinors, but do
not yet fully exploit timereversal symmetry at the manyparticle level (see Phys Rev A
2008).
The DiracCoulomb Hamiltonian or approximations to it, such as Dyall's spinfree
Hamiltonian, may be employed for rigorously determining effects of spinorbit interaction.
The method is currently capable of treating general, but small, closed and openshell molecules
and dissociation processes of heavyelement systems at high accuracy.
The CIbased version can be applied roughly up to 12 correlated
electrons whereas the correctly scaling commutatorbased version has been applied with more
than 20 correlated electrons. Both the CIbased and commutatorbased implementations have been extended
to the treatment of
electronically excited states by diagonalising the CC Jacobian. The approach corresponds to
earlier implementations of CC response theory (e.g. by Christiansen, Joergensen, and Koch)
and to EquationOfMotion (EOMCC) approaches (e.g. by Bartlett and coworkers), however
here with a fourcomponent relativistic Hamiltonian.
Much of current work deals with improving the new implementations both with respect to
efficiency and applicability.


Collaboration/Networks 

Lasse K. Sørensen and Jeppe Olsen at LCTC Aarhus, Denmark
DIRAC metalaboratory




Recent Publications
General Active Space CommutatorBased Coupled Cluster Theory of General Excitation Rank for Electronically Excited States.
Implementation and Application to ScH
Mickael Hubert, Jeppe Olsen, Jessica Loras, and Timo Fleig
J Chem Phys 139 (2013) 194106
Excitation Energies from Relativistic CoupledCluster of General Excitation Rank. Initial implementation
and application to the Si atom and the molecules XH, X={As, Sb, Bi}
Mickael Hubert, Lasse K. Sørensen, Jeppe Olsen, and Timo Fleig
Phys Rev A 86 (2012) 012503
Two and FourComponent Relativistic Generalized Active Space CoupledCluster Method.
Implementation and application to BiH
Lasse K. Sørensen, Jeppe Olsen, and Timo Fleig
J Chem Phys 134 (2011) 214102
Relativistic StringBased Electron Correlation Methods
Timo Fleig
in: Relativistic Methods for Chemists, Barysz, Maria; Ishikawa, Yasuyuki (Eds.)
Series: Challenges and Advances in Computational Chemistry and Physics, Vol. 10 (2010) 407449
A Relativistic Four and Twocomponent GeneralizedActiveSpace Coupled Cluster Method
Lasse K. Sørensen, Timo Fleig, and Jeppe Olsen
Z Phys Chem 224 (2010) 671680
Timereversal symmetry in general coupled cluster theory
Timo Fleig
Phys Rev A 77,6 (2008) 062503
A relativistic generalorder multireference coupled cluster method:
Timo Fleig, Lasse K. Sørensen, Jeppe Olsen
Theo Chem Acc 118,2 (2007) 347356
(View full list)


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