Excited State Force Constant Calculations

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 C₁ symmetry. In order to determine n, it is recommended to perform an escf calculation in C₁ symmetry. Note that numerical calculation of excited state force constants is likely to fail if there are other states nearby (in C₁), 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 C₁ 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.