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. Culpitt:2019 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.