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qp2/man/excited_states.1
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.TH "EXCITED_STATES" "1" "Jan 17, 2019" "2.0" "Quantum Package"
.SH NAME
excited_states \- | Quantum Package >
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.sp
It is possible to run excited states calculations with the quantum package. To
do this, set \fBdeterminants n_states\fP to the number of requested states.
The selection criterion will be the maximum of the selection criteria for each
state. If the Davidson diagonalization has difficulties to converge, increase
the \fBdavidson n_states_diag\fP value.
.sp
When computing multiple states, it is good to have the \fBdeterminants
s2_eig\fP flag \fBtrue\fP\&. This will force the Davidson algorithm to choose only
vectors with a value of \ewidehat{S^2} equal to \fBdeterminants expected_s2\fP\&.
Otherwise, different spin states will come out in the diagonalization.
.sp
The \fIQuantum Package\fP doesnt take account of the symmetry. Due to numerical noise, excited
states of different symmetries may enter in the calculation. Note that it is
possible to make state\-average calculation of states with different symmetries
and/or different spin multiplicities.
.sp
To include excited state of all possible symmetries, a simple trick is to
run a preliminary multi\-state CIS calculation using the CIS program,
and then running the selected FCI restarting from the CIS states, setting
\fBdeterminants read_wf\fP to \fBtrue\fP\&.
.sp
Usually, it is good practice to use state\-averaged natural MOs so that all
states have MOs of comparable quality. This allows for a faster convergence
of excitation energies.
.sp
\fBSEE ALSO:\fP
.INDENT 0.0
.INDENT 3.5
The documentation of the \fBscf()\fP, \fBcis()\fP and \fBfci()\fP programs.
.UNINDENT
.UNINDENT
.SH AUTHOR
A. Scemama, E. Giner
.SH COPYRIGHT
2018, A. Scemama, E. Giner
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