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Generic systems

read_in {
    -- options
}
Returns:
a system object.

A generic system, including atoms and molecules, can be specified by providing a file containing information about the single-particle basis set and the one- and two-body integrals between these basis functions. This file is in FCIDUMP format [Knowles89]_, which can be produced by several quantum chemistry packages including MOLPRO, Q-Chem (via additions from Alex Thom) and PSI4 (via a plugin from James Spencer). See :ref:`generating_integrals` for more details.

Options

sys

type: system object produced by a previous call.

Optional.

If provided, a previously created system object is updated with the new settings supplied, otherwise a new system object is created.

electrons

type: integer.

Optional. If specified, then ms must be specified.

Number of electrons in the unit cell. If not provided, the value in the FCIDUMP file is used.

ms

type: integer.

Optional. If specified, then electrons must be specified.

Set the spin polarisation of the system in units of electron spin (i.e. a single electron can take values 1 or -1). If not provided, the value in the FCIDUMP file is used.

sym

type: integer or string.

Optional. Default: aufbau.

Set the symmetry of the system if a reference determinant is not provided. This can be set to:

  • An integer specifying the index of a specific irreducible representation from the FCIDUMP file; see the output produced by creating a system for possible values.
  • aufbau. Uses the symmetry of a determinant selected using the Aufbau principle.
  • tot_sym. Uses the totally symmetric representation, whatever its index may be.;
Lz

type: boolean.

Optional. Default: false.

If true, enable L_z symmetry. See below for details.

int_file

type: string.

Optional. Default: 'FCIDUMP'.

Specify the FCIDUMP file containing the integrals and information relating to the single-particle basis. For details of the format see :ref:`fcidump_format`. This can also be an HDF5 file previously produced by HANDE from a FCIDUMP via the write_read_in_system function (see :ref:`utils_hdf5_system_dump`), which is both more compact in size and considerably faster to process.

dipole_int_file

type: string.

Optional. No default.

Specify a FCIDUMP-like file containing the dipole integrals, i.e. \langle i | x | i \rangle, in a given direction.

Not currently used.

CAS

type: 2D-vector of integers.

Optional. No default.

If specified, then the basis set is restricted to a given complete active space, whereby CAS = {N,M} corresponds to allowing only N electrons to be distributed among 2M spin orbitals. Any additional electrons are 'frozen' (i.e. forced to be in the lowest spin orbitals) and any additional high-energy spin orbitals are removed from the basis set.

Warning

This functionality is not compatible with reading from an HDF5 file; to use a CAS in combination with HDF5 initialisation, create the HDF5 file using a system with the desired CAS.

verbose

type: boolean.

Optional. Default: true.

Print out the single-particle basis set.

complex

type: boolean.

Optional. Default: false.

Specify if the calculation should use complex dynamics in any calculation performed, and if the FCIDUMP supplied is complex-formatted. Currently only compatible with fci and fciqmc calculations.

max_integral_chunk

type: integer

Optional. Default 2^{31} - 1.

Maximum number of MPI objects to broadcast in a single call for two body integrals. Above this value a contiguous MPI type is used instead.

Warning

This functionality is included only for ease of testing. It should not be used for production calculations.

L_z symmetry

For cylindrically symmetrical systems, the L_z (z-component of orbital angular momentum) operator commutes with the Hamiltonian, and this can be a convenient symmetry to conserve. L_z is measured in units of \hbar. Normal FCIDUMP files do not contain orbitals which are eigenfunctions of the L_z operator, so they must be transformed using post-processing. The TransLz script from the NECI project can be used for this purpose. The FCIDUMP file header format has been modified to include additional parameters: SYML, and SYMLZ which have a list of values, one for each orbital.

SYML gives the magnitude of L for the orbital if known (or -20 if not) but is not used.

SYMLZ give the eigenvalue of L_z (the m_l value). Orbitals with defined values of L_z are likely to be complex-valued, but luckily the integrals involving them are not, so althoughthe FCIDUMP file must be translated, it still retains the same format (see comments in src/read_in.F90, src/molecular_integrals.F90 and :ref:`fcidump_format` for details if you wish to create FCIDUMP files by other means).

Warning

These transformed integral files require you to enforce L_z symmetry and will produce incorrect results if you do not.