Keyword HESSIAN

This keyword specifies the choice of the starting Hessian for geometry optimization or transition state search.
Options:
BAKER / FISHER / UNITY / INTERNAL / LINDH / CALCULATE /
RESTART / READ

	BAKER		Starting Hessian constructed from Baker force weights. This is the default for the internal optimization.
	FISHER		Starting Hessian constructed from Fisher force weights. This is the default for the redundant optimization.
	UNITY		The unit matrix is used as starting Hessian. This is the default for the Cartesian optimization.
	INTERNAL		Starting Hessian from the unit matrix of an internal Hessian constructed from an auxiliary Z-matrix.
	LINDH		Starting Hessian constructed from Lindh force weights.
	CALCULATE		Starting Hessian is calculated from numerical second derivatives.
	RESTART		Starting Hessian is read from the restart file deMon.rst.
	READ		Starting Hessian is read from the input file.

PLAIN / DEMON

	PLAIN		The Hessian is read from the input file in free format row by row. Only valid in combination with the READ option.
	DEMON		The Hessian is read from the input file in the deMon2k output format. Only valid in combination with the READ option.
	VIB=$<$Real$>$		Scaling factor for the numerical step size during the calculation of second derivatives. Only valid in combination with the CALCULATE option.

Description:
With the options BAKER, FISHER, and LINDH, a starting Hessian in primitive redundant coordinates is generated, then transformed into the coordinate system used for the optimization. For the BAKER option, the diagonal elements for the primitive bonds, bends, and dihedral angles are set to 0.5, 0.2 and 0.1, respectively [194]. For the FISHER option, the diagonal elements are calculated according to the empirical rules of Fisher et al. [205]. This is the default for the starting Hessian of the default redundant optimization. With the option LINDH, the diagonal elements are calculated according to the empirical rules of Lindh et al. [206]. For a Cartesian optimization, a unit Cartesian starting Hessian is the default option. If other options are chosen for Cartesian optimizations, an auxiliary Z-matrix is constructed from the Cartesian input, and either a unit (option INTERNAL) or an empirical (options BAKER, FISHER, or LINDH) internal Hessian matrix (options BAKER, FISHER, or LINDH) is calculated. Then, this internal Hessian is transformed into the Cartesian starting Hessian.

A unit matrix starting Hessian in the space of the primitive redundant coordinates (for redundant optimizations), in the space of the internal Z-matrix coordinates (for internal optimizations), or in the space of the Cartesian coordinates is specified with the option UNITY. The option CALCULATE triggers calculation of the starting Hessian from numerical second derivatives. This is the most accurate but also the most time-consuming method. It is recommended for the transition state search. With the options RESTART and READ, the starting Hessian is read from the restart or input file respectively. The (updated) Hessian matrix is written to the restart file at the end of the optimization and also after it has been calculated using the HESSIAN or FREQUENCY keyword (see 4.8.1). With the RESTART option, that Hessian can be recovered. If the READ option is specified, the elements of the Hessian matrix must be given either in free format as real values (see example on page of the tutorial) or in the deMon2k output format (see example on page of the tutorial) in the keyword body of the HESSIAN keyword. This way the reading of the binary restart file can be bypassed. The Hessian input format is selected by the PLAIN or DEMON option. The option VIB is valid only if the option CALCULATE is also specified. For more information on the VIB option, see the keyword FREQUENCY (4.8.1).