PC GAMESS/Firefly-related discussion club

Learn how to ask questions correctly

Dmitry Ryndyk
dmitry.ryndyk@mail.ru

On Mon Nov 2 '09 5:19pm, Alex Granovsky wrote
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>Hi,

>>>However, it is still possible to perform PHVA - the simplest way
>>>is just to assign very large masses (e.g. 10⁶)
>>>to the atoms you'd like to fix.

>>That is exactly what I am doing now. :)
>>I hope that it is formally correct, but still have some doubts.
>>Because the geometry optimization is done with frozen atoms and
>>calculating the Hessian (now with all atoms) we do it in the point
>>with nonzero first derivatives.

>This approach is absolutely correct regardless on
>whether the geometry was fully or only partially optimized;
>and there are no parts in Firefly assuming that gradient
>is exactly zero when calculating second derivatives.

Thank you, that is convincing.

>>>This type of analysis is not explicitly implemented in Firefly
>>>at present. On of the reasons is that in the most cases, an overhead
>>>due to calculation of the full Hessian matrix is quite acceptable,
>>>especially for the semi-numerical calculations.

>>Well, it depends. I apply Firefly not only for isolated molecules, but also
>>for molecules coupled to metal leads and on metal surfaces. Especially
>>in the last case the large number of metal atoms should (or may) be frozen.
>>The "lost time" in this case can be longer than useful time. :(
>>

>Let us assume the Hessian has the following structure:

>|AA|AF|
>|FA|FF|

>where A is for active (i.e., non-frozen) and F is for frozen atoms.
>Even with infinitely large masses of frozen fragments,
>one still needs not only AA block, but also the entire
>off-diagonal blocks (namely |AF| and |FA| where |AF| = transpose(|FA|));
>and only |FF| block is not needed.
>This means that all these additional vibs are not completely useless at all.

>Just another interesting point - assuming the same basis on each
>atom, with half of atoms being frozen - then only 1/16 = (1-1/2)^4
>of all two electron integrals does not depend on the coordinates
>of active atoms (and hence does not depend on the changes of
>their geometry).

OK. Thank you for the detailed answer. Now I am pretty sure, that Firefly is a good choice
for this job!