Q-Chem 5.1 User’s Manual

5.4 Basic DFT Job Control

Basic SCF job control was described in Section 4.3 in the context of Hartree-Fock theory and is largely the same for DFT. The keywords METHOD and BASIS are required, although for DFT the former could be substituted by specifying EXCHANGE and CORRELATION instead.

METHOD

Specifies the exchange-correlation functional.


TYPE:

STRING


DEFAULT:

No default


OPTIONS:

NAME

Use METHOD = NAME, where NAME is either HF for Hartree-Fock theory or

 

else one of the DFT methods listed in Section 5.3.4.


RECOMMENDATION:

In general, consult the literature to guide your selection. Our recommendations for DFT are indicated in bold in Section 5.3.4.


EXCHANGE

Specifies the exchange functional (or most exchange-correlation functionals for backwards compatibility).


TYPE:

STRING


DEFAULT:

No default


OPTIONS:

NAME

Use EXCHANGE = NAME, where NAME is either:

 

1) One of the exchange functionals listed in Section 5.3.2

 

2) One of the XC functionals listed in Section 5.3.4 that is not marked with an

 

asterisk.

 

3) GEN, for a user-defined functional (see Section 5.3.6).


RECOMMENDATION:

In general, consult the literature to guide your selection. Our recommendations are indicated in bold in Sections 5.3.4 and  5.3.2.


CORRELATION

Specifies the correlation functional.


TYPE:

STRING


DEFAULT:

NONE


OPTIONS:

NAME

Use CORRELATION = NAME, where NAME is one of the correlation functionals

 

listed in Section 5.3.3.


RECOMMENDATION:

In general, consult the literature to guide your selection. Our recommendations are indicated in bold in Section 5.3.3.


The following $rem variables are related to the choice of the quadrature grid required to integrate the XC part of the functional, which does not appear in Hartree-Fock theory. DFT quadrature grids are described in Section 5.5.

FAST_XC

Controls direct variable thresholds to accelerate exchange-correlation (XC) in DFT.


TYPE:

LOGICAL


DEFAULT:

FALSE


OPTIONS:

TRUE

Turn FAST_XC on.

FALSE

Do not use FAST_XC.


RECOMMENDATION:

Caution: FAST_XC improves the speed of a DFT calculation, but may occasionally cause the SCF calculation to diverge.


XC_GRID

Specifies the type of grid to use for DFT calculations.


TYPE:

INTEGER


DEFAULT:

Functional-dependent; see Table 5.3.


OPTIONS:

0

Use SG-0 for H, C, N, and O; SG-1 for all other atoms.

$n$

Use SG-$n$ for all atoms, $n = 1, 2$, or 3

$XY$

A string of two six-digit integers $X$ and $Y$, where $X$ is the number of radial points

 

and $Y$ is the number of angular points where possible numbers of Lebedev angular

 

points, which must be an allowed value from Table 5.2 in Section 5.5.

$-XY$

Similar format for Gauss-Legendre grids, with the six-digit integer $X$ corresponding

 

to the number of radial points and the six-digit integer $Y$ providing the number of

 

Gauss-Legendre angular points, $Y = 2N^2$.


RECOMMENDATION:

Use the default unless numerical integration problems arise. Larger grids may be required for optimization and frequency calculations.


NL_GRID

Specifies the grid to use for non-local correlation.


TYPE:

INTEGER


DEFAULT:

1


OPTIONS:

Same as for XC_GRID


RECOMMENDATION:

Use the default unless computational cost becomes prohibitive, in which case SG-0 may be used. XC_GRID should generally be finer than NL_GRID.


XC_SMART_GRID

Uses SG-0 (where available) for early SCF cycles, and switches to the (larger) target grid specified by XC_GRID for final cycles of the SCF.


TYPE:

LOGICAL


DEFAULT:

FALSE


OPTIONS:

TRUE (or 1)

Use the smaller grid for the initial cycles.

FALSE (or 0)

Use the target grid for all SCF cycles.


RECOMMENDATION:

The use of the smart grid can save some time on initial SCF cycles.