[CPMD-list] restraints in BO MD: force constant kval doubled

[Martin Konopka]

Dear CPMD community,

I noticed that the restraining force constant kval is effectivelly
doubled in CPMD 3.11.1 compared to the value given in the input
file. Exploration of the code showed me that the subroutine RESTFC
is called twice in each BO MD step. Since the restraining force is
added into the total one in the manner Ftot = Ftot + Frest, it
results into effectively using 2*kval instead of kval.

If you want to use the restraints with BO MD,
possible overcomes of the problem may be:

1.) set kval in the input file to the half of the required value

2.) remove one (which one?) of the redundant calls;
One of them is directly in forces_diag.F at line 130.
The other one is most likely indirectly called in the same file
through CALL UPDWF() at line 122 of forces_diag.F .
(There is a call of FORCEDR() inside of updwf.F at its line 54.
forcedr.F itself has the other RESFC() call at line 70.)

These are only suggestions however. No warranty therefore.
I myself have been using way 1 because I have not had enough time
to test such a modification and its impact to various branches
of the code.

I only noticed that the CP MD branch does not suffer from this
problem.

Best regards
Martin.

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Dr. Martin Konopka             http://www.ccms.elf.stuba.sk/konopka.html
Department of Physics, CCMS                      tel: +421 (0)2 60291714
Slovak University of Technology (FEI STU)        fax: +421 (0)2 65411483
Ilkovicova 3, 812 19 Bratislava, Slovakia
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