energy minimization stage 1
 &cntrl
  imin=1, maxcyc=5000, ncyc=2500,
  cut=10.0, ntb=1,
  ntc=1, ntf=1,
  ntpr=10,
  ntr=1,
  restraintmask=':1-500',
  restraint_wt=2.0
 /