Nuclear Hessians are key for VPT2 calculations, as the second-order force constants calculated from them by standard harmonic-oscillator methods provide the third- and fourth-order force constants for a system via finite difference differentiation [Bar11]_ [Blo12]_.
.. todo::
Link to HO writeup once composed
In order to enable automated parsing of Hessian data from various
computational software packages, a common interface must be defined
by which the automated components of opan can access needed data in
a uniform manner. This interface is defined by the abstract
:class:`~opan.hess.SuperOpanHess` superclass, which requires that the
following members be defined for all of its subclasses:
- :attr:`hess` -- 2-D |nparray| of |npfloat| -- Hessian data in \left(\mathrm{E_h\over B^2}\right) |units|
- :attr:`geom` -- 1-D |nparray| of |npfloat| -- Geometry data in \mathrm B |units|
- :attr:`atom_syms` -- |list| of |str| -- Atomic symbols in ALL CAPS
Beyond these required members, the below subpages describe the various imported data that are available in the subclasses of :class:`~opan.hess.SuperOpanHess`.
Implemented Subclasses
.. toctree::
:maxdepth: 1
orca_hess