## Centers for orbital evaluation

Keyword: CENTERS

Description: This object represents centers around which the orbitals should be evaluated. These correspond to the centers in the ORBFILE, and the labels in the basis function objects also refer to these labels.

Required keywords

Keyword Type Description
USEATOMS Keyword Use the atomic coordinates in System.
USEGLOBAL Keyword Use the centers generated by the System. In molecular systems, this will be just the atomic centers, but in solids, it will include auxilliary centers around which to calculate the basis functions.
READ string Read the centers from a file. The format is ncenters label1 x1 y1 z1 label2 x2 y2 z2 and so on.

Optional keywords

None

## Molecular orbital evaluator

Keyword: CUTOFF_MO, STANDARD_MO, or BLAS_MO

Description: This is a collection of objects that evaluates orbitals as a linear combination of basis functions, as $\phi_i({\mathbf r})=\sum_j c_{ij} b_j({\mathbf r})$. All of these take the same input; they only differ in how they evaluate the sum. Typically BLAS_MO is the fastest implemenation if QWalk was compiled with USE_BLAS, followed by CUTOFF_MO.

Since there are often many $c_{ij}$ coefficients, these are stored in a .orb file. Its format is as follows. First there is a table with headers MO,AO(on a center),Center, Coefficient number. AO is corresponds to the basis label on a given center, that is, $b_j$. Then the keyword COEFFICIENTS, followed by the $c_{ij}$ in the order given in the table. For a complex orbital, the coefficients are in the format (real,imaginary). For example, a simple .orb file for H$_2$ in a minimal basis would be

1 1 1 1
1 1 2 2
2 1 1 3
2 1 2 4
COEFFICIENTS
0.5 0.5 0.5 -0.5


This defines two molecular orbitals, with orbital 1 bonding and orbital 2 antibonding.

Required keywords

Keyword Type Description
ORBFILE string Name of a file containing the orbital coefficients $c_{ij}$.
NMO integer Number of molecular orbitals to read from the .orb file.
BASIS section Input for a Basis function. There can be several of these sections in a row (the order matters!)

Optional keywords

Keyword Type Default Description
CENTERS section CENTERS { USEATOMS } Input for a Centers object. Possibilities include USEATOMS, USEGLOBAL (to use a set provided by the Hamiltonian), and READ to read in a centers file.
MAGNIFY float 1.0 Multiply all ( c_{ij} )\ by this factor.

## Pfaffian pairing orbital

Keyword: PAIRING_ORBITAL

Description: WARNING. This is a development feature. Make sure you know what you're doing before trying to use Pfaffians. Defines the coefficients of linear combinations of one-particle orbitals from which the pairing orbitals are constructed. The normalization is everywhere such that sum of squares of independent coefficients is equal to 1.

Required keywords

Keyword Type Description
TRIPLET_UU_COEF section N(N-1)/2 normalized coefficients for triplet spin-up spin-up pairs
TRIPLET_DD_COEF section Same as TRIPLET_UU_COEF for down-down pairs
SINGLET_COEF section N(N+1)/2 normalized coefficients for singlet pairing orbitals
UNPAIRED_COEF section Normalized coefficients for unpaired orbitals.

Optional keywords

None