| 6:39:49 PM PDT - Tue, Apr 8th 2014 |
|
Hi Paul,
My guess is the following. For the ground state the DFT code can calculate gradients analytically. So the Hessian is calculated by numerically differentiating analytic gradients. For the TDDFT we haven't released the analytic gradients yet. So now we have to calculate the Hessian by numerically differentiating numerically calculated gradients. Therefore the code now needs to perform a much larger number of finite displacements (proportional to the square of the nuclear coordinates rather than linear in the number of nuclear coordinates). It can be done but it is indeed expensive.
Huub
|