# Interface numerical_hessian

Numerical Hessian calculation by finite differences - provides Hessian for methods for which analytic calculation is not available.
Allows calculation from both energies only and from analytical gradients.

## Methods and capabilities

The interface implements a single unnamed method; 'method' keyword not necessary

## Input structure

#### The interface requires following blocks in the input:

- calculation - Setup of the actual calculation

## Keywords used

#### Keywords specific for this interface:

#### Other keywords used by this interface:

## Examples

The following examples, along with all other files needed to run them, can be found in the directory cuby4/interfaces/numerical_hessian/examples

```
#===============================================================================
# Numerical hessian example 1 - vibrational analyis
#===============================================================================
# Harmonic vibrational analysis using SCC-DFTB method which does not yield
# analytical Hessian
# Input consists of two steps, geometry optimization and vibrational analysis
job: multistep
steps: opt, freq # Names of the steps
#-------------------------------------------------------------------------------
# Geometry optimization
calculation_opt:
job: optimize
interface: dftb
method: scc-dftb
opt_quality: 0.1
history_freq: 0 # Do not save optimization history
geometry: S66:01 # Water dimer from S66 database
charge: 0
#-------------------------------------------------------------------------------
# Vibrational analysis
calculation_freq:
job: frequencies
interface: numerical_hessian
geometry: optimized.xyz # Created in teh first step
calculation: # Calculation of gradients used to construct Hessian
interface: dftb
method: scc-dftb
charge: 0
```