[CPMD-list] Force on constraint.

[Juerg Hutter]

Hi

CPMD treats fixed coordinates and intrinsic constraints differently.
Fixed coordinates are easy to handle by just ignoring the forces
acting on them. With other words the constraint force is just the
actual force on the coordinate. Intrinsic constraints, like a bond length
constraint are more difficult and need a SHAKE/RATTLE algorithm
to calculate the constraint force. It's usually this type of constraints
that is used in thermodynamic integration and it's therefore the
only forces that are written to the CONSTRAINT file.

> Dear everyone,
>
> I am doing constrained CPMD. I want to get the free energy vs. reaction
> coordinate (RC) by integrating the average force along RC. To do that, I
> printed out forces on ions at every MD step and want to get mean forces
> after time average. As a result, I found instantaneous 'forces on
> constrained(fixed) ions' are all zero in every MD step. On the other hand,
> the forces on free ions have some values but the averaged forces are closed
> to zero. My questions are,
>
> (1) Does CPMD compute forces on constrained ion, which are fixed in MD run?

Yes, see above.

> (2) If answer of (1) is yes, does CPMD reset forces on constrained ion to
> zero after computing force?

Yes, see above.

> (3) How can I get forces on the constraint?

You can print the forces before they are set to zero in forces.F.

        IF(PARENT) THEN
          CALL SYMVEC(FION,SCR,LSCR)
          CALL TAUCL(FION)             <--- "clean" forces
          CALL GSIZE(FION,GNMAX,GNORM)
        ENDIF

For example in puttau.F in subroutine taucl

      DO IS=1,NSP
        DO IA=1,NA(IS)
          IAT=IAT+1
          DO L=1,3
            IF(LSKCOR(L,IAT).EQ.0) WRITE(xx,*) L,iat,TAU(L,IA,IS)
            IF(LSKCOR(L,IAT).EQ.0) TAU(L,IA,IS)=0.0D0
          ENDDO
        ENDDO
      ENDDO

regards

Juerg Hutter

>
> Thanks for any comments in advance.
>
> Yi
> ----
> Dr. Yi Liu
> Department of Chemistry
> The University of Western Ontario
> London, Ontario, N6A 5B7, Canada
>
> Email: yliu252 at uwo.ca
> Phone: (519) 661-2111, ext. 88882
> Fax:    (519) 661-3022
>
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