MOLPRO

42 GEOMETRY OPTIMIZATION (OPTG) 308
42.1.7 Options for computing Hessians
By default, an approximate Hessian (model Hessian) is used. Optionally, a Hessian can be
computed in the optimization or read from a previous Hessian or frequency calculation.
NUMHESS=hstep
HESSREC=record
READHESS
HESSPROC=procname
HESSVAR=varname
HESSCENT
HESSFORW
If given, a numerical Hessian is computed in each hstep’th iteration.
If hstep=0 or not given, only an initial Hessian is computed.
Read initial Hessian from the given record. If record is not given or
zero, the last computed Hessian is used.
(logical). Same as HESSREC=0.
specifies a procedure to be used for computing the 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.
Compute Hessian for variable varname. This implies numerical calculation
of the Hessian from energies. The default is to use the same
variable as for the energy and gradient.
Use central gradient differences for computing Hessian (only effective
if gradients are available)
Use forward gradient differences for computing Hessian (only effective
if gradients are available). This effectively computes the Hessian
at a slightly displaced geometry, but needs only half the number of
displacements. This is the default.
UPDATE=BFGS|IBFGS|CGRD|PMS|POWELL|MS|NONE
Hessian update method to be used. See section 42.2.9 for details.
MAXUPD=maxupd
NUMDIAG
Max number of Hessian updates. The count is reset to zero each time
a Hessian is computed.
If true, replace diagonal elements of model hessian by diagonal numerical
hessian (if available). This sometimes improves convergence,
but since it may lead to symmetry breaking it is no the default.
Note that there are restrictions for computing Hessians for multireference methods (MCSCF,
MRCI, ACPF,AQCC,RS2). For these methods the symmetry must not change by any displacements,
since this could change the occupations and states and may lead to non-contiguous potential
energy surfaces. One of the following three options can be used in these cases:
• Use no symmetry from the beginning (NOSYM).
• Use symmetric displacement coordinates. This is the default if the optimization is done
in 3N cartesian coordinates. One can use OPTG,DISPLACE=SYMM to force the use of
symmetrical displacements (this creates 3N cartesian coordinates if a Z-matrix is used in
the geometry input).
• Use a Z-matrix with the restriction that no variable in the Z-matrix may change the symmetry.
For example, geometry={O;H1,O,r;H2,O,r,H1,theta} would work,
but geometry={O;H1,O,r1;H2,O,r2,H1,thetai} would not work. In this case
the program prints a warning message. If an incorrect Z-matrix is used and the symmetry
changes, the program will crash.