24. Hessians

This section relates to the computation of analytic hessians which are available for open and closed shell SCF, except ROHF and for closed shell DFT. Analytic hessians are not currently available for SCF or DFT calculations relativistic all-electron methodologies or for charge fitting with DFT. The current algorithm is fully in-core and does not use symmetry. This will be changed in the next release.

There is no required input for the Hessian module. This module only impacts the hessian calculation. For options for calculating the frequencies, please see Section 25, the Vibrational module.

24.1 Hessian Module Input

All input for the Hessian Module is optional since the default definitions are usually correct for most purposes. The generic module input begins with hessian and has the form:

  hessian
    thresh <real tol default 1d-6>
    print ...
    profile
  end

24.1.1 Defining the wavefunction threshold

You may modify the default threshold for the wavefunction. This keyword is identical to THRESH in the SCF, Section 10.7, and the CONVERGENCE gradient in the DFT, Section 11.6. The usual defaults for the convergence of the wavefunction for single point and gradient calculations is generally not tight enough for analytic hessians. Therefore, the hessian, by default, tightens these up to 1d-6 and runs an additional energy point if needed. If, during an analytic hessian calculation, you encounter an error:

cphf_solve:the available MOs do not satisfy the SCF equations

the convergence criteria of the wavefunction generally needs to be tightened.

24.1.2 Profile

The PROFILE keyword provides additional information concerning the computation times of different sections of the hessian code. Summary information is given about the maximum, minimum and average times that a particular section of the code took to complete. This is normally only useful for developers.

24.1.3 Print Control

Known controllable print options are:

Table 24.1: Hessian Print Control Specifications

Name	Print Level	Description
``hess_follow''	high	more information about where the calculation is
``cphf_cont''	debug	detailed CPHF information
``nucdd_cont''	debug	detailed nuclear contribution information
``onedd_cont''	debug	detailed one electron contribution information
``twodd_cont''	debug	detailed two electron contribution information
``fock_xc''	debug	detailed XC information during the fock builds