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Excited state vibrational frequencies can be calculated by numerical
differentiation of analytic gradients using `Numforce`
(see Chapter 11). A `Numforce` calculation for an excited state
may be started by the command

`nohup NumForce -ex `

*n*` > force.out &`

where *n* is the number of the excited state *in **C*_{1}
symmetry. In order to determine *n*, it is recommended to perform
an `escf` calculation in *C*_{1} symmetry. Note that numerical
calculation of excited state force constants *is likely to fail*
if there are other states nearby (in *C*_{1}), because the roots may
flip when the molecule is distorted. Note also that it may be
necessary to include higher excited states (using `$exopt`, see
above) in *C*_{1} calculations of molecules with higher symmetry in
order to enforce convergence to the correct state. In any case, it
should be checked that the energy change due to the displacements
(available in the `numforce/KraftWerk/*.log`

files) is reasonably
small.

For a `Numforce` run, the convergence criteria should be tightened. It
is recommended to use at least

$scfconv 8

in all `Numforce` calculations. Other `Numforce` options such as
`-central`

, `-d`

, `-np`

work in exactly the same way as
they do for ground states.

** Next:** Polarizability Derivatives and Raman
** Up:** How to Perform
** Previous:** Excited State Geometry Optimizations
** Contents**
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