[CPMD-list] P-RFO

[Salomon Billeter]

Hi,

before the transition-state search, it is very helpful to shift the atoms
most directly involved in the reaction towards the products. This has
mainly two reasons:

- Avoid starting from near a stationary point. The convergence criteria are
already met. If the system leaves the initial "stationary point" at all,
the direction you go is co-determined by the (random) residual gradient.
- At a reactant (or product) configuration, you cannot expect the curvature
along a reaction coordinate to be particularly small; if bond breaking is
involved, it's even quite unlikely that a reaction coordinate has a low
frequency there (stretching vs. bending modes). The coordinate you are
looking for could be almost anywhere in the spectrum.

Otherwise, your approach is fine for smaller systems:
- Do a vibrational analysis
- Determine the mode to follow
- Re-use the Hessian

A small technical hint: it's not enough to specify

RESTART HESSIAN

This will only affect the system Hessian which is different from the
'partial Hessian' used by P-RFO. This is required for the linear-scaling
microiterative scheme (variational decoupling: P-RFO for core / L-BFGS for
environment) where P-RFO handles only a fraction of the degrees of freedom.
However, you can copy the relevant part of the system Hessian to the
partial Hessian using

PRFO HESSTYPE
1

Also the opposite is possible if you want to use the Powell-updated Hessian
for subsequent vibrational analysis (partial Hessian to Hessian):

VIBRATIONAL ANALYSIS IN
RESTART PHESS
HESSIAN PARTIAL [UNIT,SCHLEGEL,DISCO]

Hope this helps.

Best regards
Salomon