qp2/man/cisd.1

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.TH "CISD" "1" "Mar 07, 2019" "2.0" "Quantum Package"
.SH NAME
cisd \-  | Quantum Package >
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.INDENT 3.5
Configuration Interaction with Single and Double excitations.
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This program takes a reference Slater determinant of ROHF\-like occupancy,
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and performs all single and double excitations on top of it, disregarding
spatial symmetry and compute the “n_states” lowest eigenstates of that CI
matrix (see \fBdeterminants n_states\fP).
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This program can be useful in many cases:
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.IP \(bu 2
\fBGround state calculation\fP: if even after a \fBcis()\fP calculation, natural
orbitals (see \fBsave_natorb()\fP) and then \fBscf()\fP optimization, you are not sure to have the lowest scf
solution,
do the same strategy with the \fBcisd()\fP executable instead of the \fBcis()\fP\ exectuable to generate the natural
orbitals as a guess for the \fBscf()\fP\&.
.IP \(bu 2
\fBExcited states calculations\fP: the lowest excited states are much likely to
be dominanted by single\- or double\-excitations.
Therefore, running a \fBcisd()\fP will save the “n_states” lowest states within
the CISD space
in the \fI\%EZFIO\fP directory, which can afterward be used as guess wave functions
for a further multi\-state fci calculation if you specify “read_wf” = True
before running the fci executable (see \fBdeterminants read_wf\fP).
Also, if you specify “s2_eig” = True, the cisd will only retain states
having the good value S^2 value
(see \fBdeterminants expected_s2\fP and \fBdeterminants s2_eig\fP).
If “s2_eig” = False, it will take the lowest n_states, whatever
multiplicity they are.
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Note: if you would like to discard some orbitals, use
qp_set_mo_class to specify:
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.IP \(bu 2
“core” orbitals which will be always doubly occupied
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“act” orbitals where an electron can be either excited from or to
.IP \(bu 2
“del” orbitals which will be never occupied
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.IP \(bu 2
\fBread_wf\fP
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Calls:
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.IP \(bu 2
\fBrun()\fP
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Touches:
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.IP \(bu 2
\fBfock_matrix_ao_alpha\fP
.IP \(bu 2
\fBfock_matrix_ao_alpha\fP
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\fBmo_coef\fP
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\fBlevel_shift\fP
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.IP \(bu 2
\fBmo_coef\fP
.IP \(bu 2
\fBread_wf\fP
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.SH AUTHOR
A. Scemama, E. Giner
.SH COPYRIGHT
2019, A. Scemama, E. Giner
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