********> bugfix.57
Correction Author: David A. Pearlman
Correction Date: 04/09/93

Programs: GIBBS

Severity: Moderate

Problem: 

       If 
     
       1) Thermodynamic Integration is being used to calculate
          free energies (IDIFRG=1); and

       2) The contributions of pert-group internals to the free energy
          are being determined (INTPRT=3 or INTPRT=5);

       the contributions due to perturbed torsions will be incorrectly
       calculated. On many platforms, the contribution will be
       reported as 0.0. On other platforms (e.g. hp), the contribution
       will be reported as a floating overflow value (a clear indication
       that something is wrong!)

       This problem does NOT affect calculations performed using
       Free Energy Perturbation (FEP; the default method), nor will
       there be any affect if the torsional contributions in TI are
       not determined.

Affects: 

       Depending on how the host computer treats uninitialized variables,
       the reported contribution due to torsions (part of BADH) will
       probably be either 0.0 or an extrememly large value.

Cause: 

       A variable was incorrectly being used before it was set. The pointer
       into several arrays was also being incorrectly set.

Fix:  Make the following changes to routine PEPHI

---To routine PEPHI in file gibb.f -----------------------------------------

*** OLD gibb.f
--- NEW gibb.f
***************
*** 3627,3632 ****
--- 3627,3633 ----
        DF = CT*(CT2*(CT2*192.0-192.0)+36.0)
  C
     60 ARG = PK(IC)
+       IOLDFM = 0
        IF(PHASE(IC).EQ.0.0) GO TO 65
        ARG = -ARG
  C
***************
*** 3641,3646 ****
--- 3642,3648 ----
        SARGUM = SIN(ARGUM)
        DFP = -APN*SARGUM
        DF = -DFP/SPHI
+       IOLDFM = 1
        WRITE(6,1530) I3,J3,K3,L3,PN(IC),PHASE(IC)
  C
     65 E = PK(IC)+ARG*E
***************
*** 3661,3669 ****
  C negligable, and saves us a lot of extra calculations here.
  C
        IF (IDIFRG.EQ.1 .AND. KPERT.EQ.2) THEN
!          DN = ABS(PN(IPHI)) - ABS(PN(IC))
!          DPHI = PHASE(IPHI) - PHASE(IC)
!          RKU = ALM(1)*PK(IPHI) + ALM(2)*PK(IC)
  C
  C Define for/rev values of K (RKU), N (RN) and PHI (RPHI)
  C
--- 3663,3673 ----
  C negligable, and saves us a lot of extra calculations here.
  C
        IF (IDIFRG.EQ.1 .AND. KPERT.EQ.2) THEN
!          ICOLD = ICP(IPHI)
!          DN = ABS(PN(ICOLD)) - ABS(PN(IC))
!          DPHI = PHASE(ICOLD) - PHASE(IC)
!          RKU = ALM(1)*PK(ICOLD) + ALM(2)*PK(IC)
!          IF (IOLDFM.EQ.0) SARGUM = SIN(ABS(PN(IC))*AP-PHASE(IC))
  C
  C Define for/rev values of K (RKU), N (RN) and PHI (RPHI)
  C
------------------------------------------------------------------------------


Temporary workarounds: 

     None.

Routines affected:

     GIBBS   Routine PEPHI in file ...amber4/src/gibbs/gibb.f