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Overview

Functionality

  • Calculate \DeltaSCF-DFT excitation energies by changing band occupation - the "simple" method.
  • Constrain and occupy an orbital of a subsystem or reference system to resemble an electronic excitation via 'linear expansion mode' = le \DeltaSCF-DFT
  • Put penalties onto orbitals of a subsystem in a DFT+U fashion (DFT+U(MO)). This can be done by enforcing idempotency (integer occupation) or by constrained DFT
  • Generate a projection of the orbital of a subsystem on the Density-of-States (DOS)
  • There is also a separate post-processing tool (MolPDOS) to generate the corresponding DOS from the projection information

Prerequisites

  • The following applies to all modes, except the \DeltaSCF-DFT mode. Nonetheless, this runmode requires a good understanding of its limitations.

  • The two systems should not be strongly geometrically or electronically mixed and a separation into general system and subsystem has to still be chemically reasonable. Otherwise this Ansatz breaks down. Ideally the difference can be seen as a weak perturbation. This applies for example for

    • A molecule adsorbed at a metal surface with weak to medium hybridization, e.g. C6H6\@Au(111).
    • A particle or molecule inserted into a porous nanostructure.
    • The exact same system in another electronic state, e.g. groundstate vs. first excited state.

  • The wavefunction of the subsystem has to be calculated with exactly the same settings (K-points, spin-polarisation, cutoff, cell size) as the actual system. This introduces some artificial dispersion for gas-phase molecules, but if the cell is sufficiently large the effects should be small. In addition, this might be something desirable, for example when high coverage situations are investigated.

  • At the moment, the excitation constraints only work for metals_method=DM and spin_treatment=scalar or none. Currently, hybrid XC functionals are unsupported.

General Use

The general use of \DeltaSCF always involves the following steps:

  1. Define the base system and calculate the self consistent density and wave functions using a task=single point energy. The checkfile generated is then used as the base for the next steps.

  2. Define the system you want to study and add something similar to the following to the<seed>.param file:

reuse               : <base>.check
calculate_deltascf  : true
deltascf_checkpoint : <base>.check
deltascf_mode       :  1

where <base> is the seedname of the base calculation in step 1.

  1. Decide on the type of \DeltaSCF calculation. The allowed methods are:
deltascf_method : SIMPLE        or  deltascf_mode : 1
deltascf_method : DFT+U(MO)     or  deltascf_mode : 2
deltascf_method : LINEAR EXPANSTION or  deltascf_mode : 3

For each method, the constraints need to be specified as follows:

SIMPLE 

#band  occ spin from_band to_band
%block deltascf_constraints
5    0.5000  1   5   5
6    0.5000  1   6   6
%endblock deltascf_constraints

DFT+U(MO) 

#band  occ spin U(eV) excite_band
%block deltascf_constraints
5    0.5000  1   +0.0   .true.
6    0.5000  1   +0.0   .true.
%endblock deltascf_constraints

LINEAR EXPANSION 

#band  occ spin
%block deltascf_constraints
5    0.5000  1
6    0.5000  1
%endblock deltascf_constraints

The "occ" column is the constrained occupation required. In this instance, 1 electron is being raised from band 5 to band 6. The occupation is defined such that 0<=occ<=1 so if non-spin polarized calculation then occ=0.5 means 1 electron.

  1. You can now run the \DeltaSCF calculation as any other CASTEP task.

  2. If you wish to, you can analyse the results using the MolPDOS post-processing tool which needs an additional <seed>.molpdos input file. Hence use MolPDOS <seed>