********> bugfix.37
Correction Author: David A. Pearlman
Correction Date: 3/10/92  

Programs: SANDER

Severity: moderate

Problem: 

       When using time-averaged restraints, if you explicitly set
       the characteristic time for exponential decay (VALUE1) to 0.0,
       this value will not default to a very large value. The manual claims 
       this will be the default used when VALUE1 is set to 0.0. (Such a
       large value would result in no exponential decay weighting).
       Instead, a value of 1.0D-7 will be used, which will result in
       precipitous exponential decay.

       If VALUE1 is not specified in the &wt namelist where 
       TYPE=(DISAVE, ANGAVE or TORAVE), or if VALUE1 is explicitly set
       to any non-0 value, the program will work correctly. In a formatted
       style weight change line with TYPE=(DISAVE, ANGAVE or TORAVE) one
       *must* set VALUE1 to a non-zero value.

Affects: 
       The md trajectory may become unstable. Alternatively, you may
       observe that the restraint violations appear to be increasing
       (or at least not decreasing) as the simulation progresses.

Cause: 
        
       The default value was not being set in accordence with the manual.

Fix:  Make the following change

***To routine NMRRED in src/sander/nmrred.f:

*** OLD nmrred.f
--- NEW nmrred.f
***************
*** 495,500
  C             IF (NEXACT(JDX).EQ.0) NEXACT(JDX) = MAX(IMULT,1)
                IF (IPOWER(JDX).EQ.0) IPOWER(JDX) = NINT(VALUE2)
                IMULT = 0
                VALUE2 = VALUE1
            END IF
  C

--- 495,501 -----
  C             IF (NEXACT(JDX).EQ.0) NEXACT(JDX) = MAX(IMULT,1)
                IF (IPOWER(JDX).EQ.0) IPOWER(JDX) = NINT(VALUE2)
                IMULT = 0
+               IF (VALUE1.LT.ZERNER) VALUE1 = 1.0D+7
                VALUE2 = VALUE1
            END IF
  C

Temporary workarounds: 
     If you want the to force no exponential decay weighting, set
     VALUE1 to a value .GE.1.0D+6.

Routines affected:

     SANDER  Routine NMRRED in  .../amber4/sander/nmrred.f