MOLPRO

42 GEOMETRY OPTIMIZATION (OPTG) 313
If the Model Hessian is disabled (NOMODEL) and no Hessian is read or computed, the initial
hessian is assumed to be diagonal, with values 1 hartree*bohr**(-2) for all lengths, 1
hartree*radian**(-2) for all angles. Additional matrix elements of the hessian can be defined
using the HESSELEM directive, see section 42.2.8.
In transition state searches the Hessian is evaluated numerically in the first iteration by default.
Alternatively, if READ is specified, a previously computed hessian is used.
42.2.7 Numerical Hessian (NUMHESS)
NUMHESS,options
or
NUMHESS,hstep,options
If this directive is present a numerical Hessian is computed using finite differences. If analytical
gradients are available, one can use forward gradient differences (needs one gradient calculation
for each coordinate) or central differences (more accurate, needs two gradient calculations
for each coordinate). For transition state optimizations it is usually sufficient to use forward
differences. If analytical gradients are not available for the optimized method, the energy is
differentiated twice. In this case only central differences are possible.
The following options can be given:
HSTEP=hstep
FORWARD
CENTRAL
hstep=-1: Don’t calculate numerical hessian (default for minimization);
hstep=0 Calculate numerical hessian only once at the start of the optimization
(default for transition state searches).
hstep=n Calculate numerical hessian after each n optimization steps.
This is useful for difficult transition state optimizations (e.g. if the
eigenvalue structure of the hessian changes during the optimization).
Use forward differences (default).
Use the more accurate central differences.
RSTEP=rstep Step length for distances (in bohr). The default is 0.01.
ASTEP=astep
DSTEP=dstep
VARIABLE=varname
Step length for angles (in degree). The default is 0.5 or 1 for angles
below and above 90 degree, respectively.
Step length for symmetrical displacements (in bohr). The default is
0.01.
Use given variable for numerical calculation of the Hessian. Note that
this disables the use of gradients, and Hessian evaluation can be very
expensive.
PROCEDURE=procname Procedure to be used for computing Hessian. This procedure must be
define a complete energy calculation (orbital optimization and correlation
treatment). A different method can be used than for the optimized
energy. For instance, an MP2 hessian can be used for CCSD(T)
optimizations, or a CASPT2 hessian for MRCI optimizations. By default,
the same procedure is used for the hessian as for the optimized
energy.
DISPLACE=type
type can be one of the following: