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(June 30, 2021)

The NEO method is a natural extension of the Self-Consistent-Field methods and it inherits most of its functionalities. Thus, the keywords that are used in the SCF Job Control are used in the NEO-SCF methods with a few additional keywords. The NEO-SCF methods require definition of the nuclear basis sets (see Examples for more information). Refer to Ref. 234 for selection of the protonic basis sets. Only pure (Spherical) Gaussian basis sets are currently available.
The following three *$rem* variables must be specified in order to run NEO-SCF calculations:

NEO

Enable a NEO-SCF calculation.

TYPE:

BOOLEAN

DEFAULT:

FALSE

OPTIONS:

TRUE
Enable a NEO-SCF calculation.
FALSE
Disable a NEO-SCF calculation.

RECOMMENDATION:

Set to TRUE if desired.

METHOD

Specifies the exchange-correlation functional.

TYPE:

STRING

DEFAULT:

No default

OPTIONS:

*NAME*
Use METHOD = *NAME*, where *NAME* is one of the following:
HF for Hartree-Fock theory;
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.

BASIS

Specifies the electronic basis sets to be used.

TYPE:

STRING

DEFAULT:

No default basis set

OPTIONS:

General, Gen
User defined (*$basis* keyword required).
Symbol
Use standard basis sets as per Chapter 8.
Mixed
Use a mixture of basis sets (see Chapter 8).

RECOMMENDATION:

Consult literature and reviews to aid your selection.

In addition, the following *$rem* variables, that appear in the conventional SCF calculations can be used to customize the NEO-SCF calculation:

SCF_CONVERGENCE

NEO-SCF is considered converged when the electronic wave function error is less that
${10}^{-\mathrm{SCF}\mathrm{\_}\mathrm{CONVERGENCE}}$. Adjust the value of THRESH at the same
time. (Starting with Q-Chem 3.0, the DIIS error is measured by the maximum error
rather than the RMS error as in earlier versions.)

TYPE:

INTEGER

DEFAULT:

5
For single point energy calculations.
8
For geometry optimizations.

OPTIONS:

User-defined

RECOMMENDATION:

None.

NEO_N_SCF_CONVERGENCE

NEO-SCF is considered converged when the nuclear wave function error is less that
${10}^{-\mathrm{NEO}\mathrm{\_}\mathrm{N}\mathrm{\_}\mathrm{SCF}\mathrm{\_}\mathrm{CONVERGENCE}}$.

TYPE:

INTEGER

DEFAULT:

7

OPTIONS:

User-defined

RECOMMENDATION:

None.

UNRESTRICTED

Controls the use of restricted or unrestricted orbitals.

TYPE:

LOGICAL

DEFAULT:

FALSE
Closed-shell systems.
TRUE
Open-shell systems.

OPTIONS:

FALSE
Constrain the spatial part of the alpha and beta orbitals to be the same.
TRUE
Do not Constrain the spatial part of the alpha and beta orbitals.

RECOMMENDATION:

The ROHF method is not available. Note that for unrestricted calculations on
systems with an even number of electrons it is usually necessary to break
$\alpha $/$\beta $ symmetry in the initial guess, by using SCF_GUESS_MIX or
providing *$occupied* information (see Section 4.4 on initial guesses).

MAX_SCF_CYCLES

Controls the maximum number of SCF iterations permitted.

TYPE:

INTEGER

DEFAULT:

50

OPTIONS:

$n$
$n>0$ User-selected.

RECOMMENDATION:

Increase for slowly converging systems such as those containing transition
metals.

SCF_ALGORITHM

Algorithm used for converging the SCF.

TYPE:

STRING

DEFAULT:

DIIS
Pulay DIIS.

OPTIONS:

DIIS
Pulay DIIS.
DM
Direct minimizer.
DIIS_DM
Uses DIIS initially, switching to direct minimizer for later iterations
(See THRESH_DIIS_SWITCH, MAX_DIIS_CYCLES).
DIIS_GDM
Use DIIS and then later switch to geometric direct minimization
(See THRESH_DIIS_SWITCH, MAX_DIIS_CYCLES).
GDM
Geometric Direct Minimization.
RCA
Relaxed constraint algorithm
RCA_DIIS
Use RCA initially, switching to DIIS for later iterations (see
THRESH_RCA_SWITCH and MAX_RCA_CYCLES described
later in this chapter)
ROOTHAAN
Roothaan repeated diagonalization.

RECOMMENDATION:

In the NEO methods, the GDM procedure is recommended.

JOBTYPE

Specifies the calculation.

TYPE:

STRING

DEFAULT:

Default is single-point, which should be changed to one of the following options.

OPTIONS:

OPT
Equilibrium structure optimization.
TS
Transition structure optimization is currently not available in NEO.
RPATH
Intrinsic reaction path following is currently not available in NEO.

RECOMMENDATION:

Application-dependent. Always use SYM_IGNORE = 1 with geometry optimization.

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, $Y=2{N}^{2}$.

RECOMMENDATION:

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

Additional NEO specific *$rem* variables can be used to customize the NEO-SCF calculation:

NEO_E_CONV

Energy convergence criteria in the NEO-SCF calculations so that the difference in energy between electronic and protonic iterations is less than ${10}^{-\mathrm{NEO}\mathrm{\_}\mathrm{E}\mathrm{\_}\mathrm{CONV}}$.

TYPE:

INTEGER

DEFAULT:

8

OPTIONS:

User-defined

RECOMMENDATION:

Tighter criteria for geometry optimization are recommended.

NEO_BASIS_LIN_DEP_THRESH

This keyword is used to set the liner dependency threshold for nuclear basis sets. It is defined as ${10}^{-\mathrm{NEO}\mathrm{\_}\mathrm{BASIS}\mathrm{\_}\mathrm{LIN}\mathrm{\_}\mathrm{DEP}\mathrm{\_}\mathrm{THRESH}}$.

TYPE:

DOUBLE

DEFAULT:

5.0

OPTIONS:

User-defined

RECOMMENDATION:

No recommendation.

NEO_PURECART

This keyword is used to specify Cartesian or spherical Gaussians for nuclear basis functions.

TYPE:

INTEGER

DEFAULT:

2222

OPTIONS:

User-defined

RECOMMENDATION:

Default are Cartesian Gaussians. 1111 would define spherical Gaussians similar to keyword PURECART. Current NEO calculations do not support Cartesian electronic or nuclear basis sets with h angular momentum.

NEO_ISOTOPE

Enable calculations of different types of isotopes. Only one type of isotope is allowed at present.

TYPE:

INTEGER

DEFAULT:

1
Default is the proton isotope.

OPTIONS:

1
This NEO calculation is using proton isotope.
2
This NEO calculation is using deuterium isotope.
3
This NEO calculation is using tritium isotope.

RECOMMENDATION:

Refer to the NEO literature for the best performance on the isotope effects calculations.

NEO_VPP

Remove $J-K$ terms from the nuclear Fock matrix and the corresponding kernel terms for NEO excited state methods for the case of one quantum proton.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

1
Enable this option.
0
Disable this option.

RECOMMENDATION:

Use this only in the case of one quantum hydrogen.

NEO_EPC

Specifies the electron-proton correlation functional.

TYPE:

STRING

DEFAULT:

No default

OPTIONS:

*NAME*
Use NEO_EPC = *NAME*, where *NAME* can be either epc172 or epc19.

RECOMMENDATION:

Consult the NEO literature to guide your selection.

The following additional *$rem* variables can be used to customize the NEO excited states methods calculation to obtain excitation energies:

SET_ROOTS

Sets the number of NEO excited state roots to find by Davidson or display the number of roots obtained by direct diagonalization.

TYPE:

INTEGER

DEFAULT:

0
Do not look for any excited states.

OPTIONS:

$n$
$n>0$ Looks for $n$ NEO excited states.

RECOMMENDATION:

None

SET_RPA

Do a NEO-TDDFT or NEO-TDHF calculation.

TYPE:

LOGICAL/INTEGER

DEFAULT:

FALSE

OPTIONS:

FALSE
Do a NEO-TDA or NEO-CIS calculation.
TRUE
Do a NEO-TDDFT or NEO-TDHF calculation.

RECOMMENDATION:

Consult the NEO literature to guide your selection.

DIRECT_DIAG

Perform direct diagonalization to obtain all the NEO excitation energies.

TYPE:

INTEGER

DEFAULT:

0
Use Davidson algorithm.

OPTIONS:

1
Do the direct diagonalization.
0
Use Davidson algorithm.

RECOMMENDATION:

Only use this option when Davidson solutions are not stable.