A form of inversion potential has been encountered which
DL_POLY_2 does not recognise.
Action:
Locate the offending potential in the FIELD file and
remove. Replace with one acceptable to DL_POLY_2 if this is
reasonable. Alternatively, you may consider defining the required
potential in the code yourself. Amendments to subroutines SYSDEF
and INVFRC will be required.