problems of improper dihedral calculation with Fist

[姚懿]

In the mol_force.F file. there is one line in force_imp_torsions subroutine

phi = SIGN( ACOS ( cosphi ), DOT_PRODUCT ( t12, tn ))

I suggest to change it to 

phi = ACOS( cosphi )

since improper potential is not periodic. For a dihedral close to 180 
degree it can varies between -180.0 and 180.0 that will influence the 
result a lot.

YY
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