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.TH "EXCITED_STATES" "1" "Jun 15, 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 doesn’t 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 states 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 scf, cis and
fci programs.
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.SH AUTHOR
A. Scemama, E. Giner
.SH COPYRIGHT
2019, A. Scemama, E. Giner
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