********> bugfix.72
Author:   Bill Ross
Date:     4/23/96

Programs: Prep

Severity: Slight

Problem:  Use of '-M' in IMPROPER specification is restricted to
          the 1st 2 atoms; bad 1-4 nonbond energies result from
          use for 4th atom, and 3rd atom is inappropriate. This
          fix adds a trap for this form of input error. Also, the
          manual does not explain use of -M and +M. Explanation
          updated in web version of manual as follows:

--
        'IMPROPER'  Control for reading the improper torsion angles.  A
              proper torsion I - J - K - L has I bonded to J bonded
              to K bonded to L.  An IMPROPER torsion is any torsion in
              which this is not the case.  Improper torsions are used to
              keep the asymmetric centers from racemizing in the united
              atom model where all the C-H hydrogens are omitted.  They
              can also be used to enforce planarity.  The normal case is:

                                 J
                                 |
                                 K
                                / \
                               I   L

                          Improper I-J-K-L

              where the central atom (K) is the third atom in the improper
              and the order of the other three is determined alphabetically
              by atom type and if types are the same by atom number.
              The improper torsions should be defined in such a way that
              the proper torsions are not duplicated.  The atoms making the
              improper torsions are read as their atom (IGRAPH) names.
              '-M' can be used in place of an atom name to indicate the
              last main chain atom in the previous residue, and '+M' for
              the first main chain atom in the next residue. NOTE: -M
              cannot be used in the 3rd or 4th positions ('K' and 'L')
              owing to internal data representation limitations.
              A BLANK CARD terminates this section.
--

Temporary workarounds: 

              None - this fix only catches the error in prep.


Fix:  Make the following change to prep.f:
---------------------------------------------------------------------------
*** OLD prep.f
--- NEW prep.f
***************
*** 767,772
            CALL FINDN(NATOM,MNB(J),INUM2,IGRAPH)
            CALL FINDN(NATOM,MNC(J),INUM3,IGRAPH)
            CALL FINDN(NATOM,MND(J),INUM4,IGRAPH)
            IF(INUM1.EQ.-99.OR.INUM2.EQ.-99.OR.INUM3.EQ.-99.OR.
       +         INUM4.EQ.-99) then
              WRITE(IOUT,9108) MNA(J),MNB(J),MNC(J),MND(J)

--- 767,784 -----
            CALL FINDN(NATOM,MNB(J),INUM2,IGRAPH)
            CALL FINDN(NATOM,MNC(J),INUM3,IGRAPH)
            CALL FINDN(NATOM,MND(J),INUM4,IGRAPH)
+           if (INUM3.eq.-1) then
+             write(iout, '(1x,a,a)') 
+      +          ' Error: -M cannot be used in 3rd position',
+      +          ' for impropers'
+             call mexit(iout, 1)
+           endif
+           if (INUM4.eq.-1) then
+             write(iout, '(1x,a,a)') 
+      +          ' Error: -M cannot be used in 4th position',
+      +          ' for impropers'
+             call mexit(iout, 1)
+           endif
            IF(INUM1.EQ.-99.OR.INUM2.EQ.-99.OR.INUM3.EQ.-99.OR.
       +         INUM4.EQ.-99) then
              WRITE(IOUT,9108) MNA(J),MNB(J),MNC(J),MND(J)
---------------------------------------------------------------------------

--