$DRC group                   (relevant for RUNTYP=DRC)
 
   This group governs "direct dynamics", following the
dynamical reaction coordinate, which is a classical
trajectory based on quantum chemistry potential energy
surfaces.  These may be either ab initio or semi-empirical,
and are computed "on the fly" as the trajectory proceeds.
 
   Because the vibrational period of a normal mode with
frequency 500 wavenumbers is 67 fs, a DRC needs to run for
many steps in order to sample a representative portion of
phase space.  Restart data can be found in the job's OUTPUT
file, with important results summarized to the TRAJECT
file.  Almost all DRCs break molecular symmetry, so build
your molecule with C1 symmetry in $DATA, or specify NOSYM=1
in $CONTRL.  RUNTYP=DRC may not be used with EFP particles.
 
NSTEP  = The number of DRC points to be calculated, not
         including the initial point.  (default = 1000)
 
DELTAT = is the time step.  (default = 0.1 fs)
 
TOTIME = total duration of the DRC computed in a previous
         job, in fs.  The default is the correct value
         when initiating a DRC.  (default=0.0 fs)
 
                           * * *
 
      In general, a DRC can be initiated anywhere,
      so $DATA might contain coordinates of the
      equilibrium geometry, or a nearby transition
      state, or something else.  You must also
      supply an initial kinetic energy, and the
      direction of the initial velocity, for which
      there are a number of options:
 
EKIN   = The initial kinetic energy (default = 0.0
kcal/mol)
         See also ENM, NVEL, and VIBLVL regarding alternate
         ways to specify the initial value.
 
VEL    = an array of velocity components, in Bohr/fs.
         When NVEL is false, this is simply the direction
         of the velocity vector.  Its magnitude will be
         automatically adjusted to match the desired
initial
         kinetic energy, and it will be projected so that
         the translation of the center of mass is removed.
         Give in the order vx1, vy1, vz1, vx2, vy2, ...
 
NVEL   = a flag to compute the initial kinetic energy from
         the input VEL using the sum of mass*VEL*VEL/2.
         This flag is usually selected only for restarts.
         (default=.FALSE.)
 
 
         The next three allow the kinetic energy to be
         partitioned over all normal modes.  The
         coordinates in $DATA are likely to be from
         a stationary point!  You must also supply $HESS
         input, which is the nuclear force constant
         matrix at the starting geometry.
 
VIBLVL = a flag to turn this option on (default=.FALSE.)
 
VIBENG = an array of energies (in units of multiples of
         the hv of each mode) to be imparted along each
         normal mode.  The default is to assign the zero
         point energy only, VIBENG(1)=0.5, 0.5, ..., 0.5
         when HESS=MIN, and 0.0, 0.5, ..., 0.5 if HESS=TS.
         If given as a negative number, the initial
         direction of the velocity vector is along the
         reverse direction of the mode.  "Reverse" means
         the phase of the normal mode is chosen such that
         the largest magnitude component is a negative
         value.  An example might be VIBENG(4)=2.5 to add
         two quanta to mode 4, along with zero point
         energy in all modes.
 
RCENG  = reaction coordinate energy, in kcal/mol.  This is
         the initial kinetic energy given to the imaginary
         frequency normal mode when HESS=TS.  If this is
         given as a negative value, the direction of the
         velocity vector will be the "reverse direction",
         meaning the phase of the normal mode will be
         chosen so its largest component is negative.
 
                           * * *
 
         The next two pertain to initiating the DRC along
         a single normal mode of vibration.  No kinetic
         energy is assigned to the other modes.  You must
         also supply $HESS input for the initial geometry.
 
NNM    = The number of the normal mode to which the initial
         kinetic energy is given. The absolute value of NNM
         must be in the range 1, 2, ..., 3N-6.  If NNM is a
         positive/negative value, the initial velocity will
         lie in the forward/reverse direction of the mode.
         "Forward" means the largest normal mode component
         is a positive value.  (default=0)
 
ENM    = the initial kinetic energy given to mode NNM,
         in units of vibrational quanta hv, so the amount
         depends on mode NNM's vibrational frequency, v.
         If you prefer to impart an arbitrary initial
         kinetic energy to mode NNM, specify EKIN instead.
         (default = 0.0 quanta)
 
To summarize, there are 5 ways to initiate a trajectory:
 
   1. VEL vector with NVEL=.TRUE.  This is difficult to
      specify at your initial point, and so this option
      is mainly used when restarting your trajectory.
      The restart information is always in this format.
   2. VEL vector and EKIN with NVEL=.FALSE.  This will
      give a desired amount of kinetic energy in the
      direction of the velocity vector.
   3. VIBLVL and VIBENG and possibly RCENG, to give some
      initial kinetic energy to all normal modes.
   4. NNM and ENM to give quanta to a single normal mode.
   5. NNM and EKIN to give arbitrary kinetic energy to
      a single normal mode.
 
                           * * *
 
       The most common use of the next two is to analyze
       a trajectory with respect to the normal modes of
       a minimum energy geometry it travels around.
 
NMANAL = a flag to select mapping of the mass-weighted
         Cartesian DRC coordinates and velocity (conjugate
         momentum) in terms of normal modes at a nearby
         reference stationary point (which can be either a
         minimum or transition state).  This reference
         geometry could in fact be the same as the initial
         point of the DRC, but does not need to be.
         If you choose this option, you must supply C0,
         HESS2, and $HESS2 input corresponding to the
         reference stationary point.  (default=.FALSE.)
 
C0     = an array of the coordinates of the stationary
         reference point (the coordinates in $DATA might
         well be some other coordinates).  Give in the
         order x1,y1,z1,x2,y2,... in Angstroms.
 
                           * * *
 
       The next options apply to input choices which may
       read a $HESS at the initial DRC point, namely NNM
       or VIBLVL, or to those that read a $HESS2 at some
       reference geometry (NMANAL).
 
HESS   = MIN indicates the hessian supplied for the initial
             geometry corresponds to a minimum (default).
       = TS  indicates the hessian is for a saddle point.
HESS2  = MIN (default) or TS, the same meaning, for the
         reference geometry.
 
      These are used to decide if modes 1-6 (minimum) or
      modes 2-7 (TS) are to be excluded from the hessian
      as the translational and rotational contaminants.
      If the initial and reference geometries are the same,
      these two hessians will be duplicates of each other.
 
 
    The next variables can cause termination of a run, if
molecular fragments get too far apart or close together.
 
NFRGPR = Number of atom pairs whose distance will be
         checked.  (default is 0)
 
IFRGPR = Array of the atom pairs.  2 times NFRGPR values.
 
FRGCUT = Array for a boundary distance (in Bohr) for atom
         pairs to end DRC calculations.  The run will
         stop if any distance exceeds the tolerance, or if
         a value is given as a negative number, if the
         distance becomes shorter than the absolute value.
         In case the trajectory starts outside the bounds
         specified, they do not apply until after the
         trajectory reaches a point where the criteria
         are satisfied, and then goes outside again.
         Give NFRGPR values.
 
                           * * *
 
    The final variables control the volume of output.
    Let F mean the first DRC point found in this run,
    and L mean the last DRC point of this run.
 
NPRTSM = summarize the DRC results every NPRTSM steps,
         to the TRAJECT file.  (default = 1)
 
NPRT   =  1  Print orbitals at all DRC points
          0  Print orbitals at F+L (default)
         -1  Never print orbitals
 
NPUN   =  2  Punch all orbitals at all DRC points
          1  Punch all orbitals at F+L, and occupied
             orbitals at DRC points between
          0  Punch all orbitals at F+L only (default)
         -1  Never punch orbitals
 
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