B.1.2 Descriptions of Some Q-Chem Input Sections

Four methods are available for inputing geometry information:

• •

  Z-matrix (Ångstroms and degrees):
  $molecule
        [charge] [multiplicity]
        [Z-matrix]
        [blank line, if parameters are being used]
        [Z-matrix parameters, if used]
  $end
  

• •

  Cartesian Coordinates (Ångstroms):
  $molecule
        [charge] [multiplicity]
        [Cartesian coordinates]
        [blank line, if parameter are being used]
        [Coordinate parameters, if used]
  $end
  

• •

  Read from a previous calculation:
  $molecule
        read
  $end
  

• •

  Read from a file:
  $molecule
        read filename
  $end
  

See also the list of $rem variables at the end of this Appendix. The general format is:

$rem
   REM_VARIABLE   VALUE   [optional comment]
$end

although specifying “REM_VARIABLE = VALUE” is also acceptable, i.e., the equals sign is ignored.

The format for the user–defined basis section is as follows:

$basis
	$X$	0
	$L$	$K$	$scale$
	${\alpha }_1$	$C_1^{L_{\text{min}}}$	$C_1^{L_{\text{min}}+1}$	$\mathrm{\dots }$	$C_1^{L_{\text{max}}}$
	${\alpha }_2$	$C_2^{L_{\text{min}}}$	$C_2^{L_{\text{min}}+1}$	$\mathrm{\dots }$	$C_2^{L_{\text{max}}}$
	$\mathrm{⋮}$	$\mathrm{⋮}$	$\mathrm{⋮}$	$\mathrm{\ddots }$	$\mathrm{⋮}$
	${\alpha }_K$	$C_K^{L_{\text{min}}}$	$C_K^{L_{\text{min}}+1}$	$\mathrm{\dots }$	$C_K^{L_{\text{max}}}$
****
$end

where $X$ Atomic symbol of the atom (atomic number not accepted) $L$ Angular momentum symbol (S, P, SP, D, F, G) $K$ Degree of contraction of the shell (integer) $scale$ Scaling to be applied to exponents (default is 1.00) $a_i$ Gaussian primitive exponent (positive real number) $C_i^L$ Contraction coefficient for each angular momentum (non–zero real numbers).

Atoms are terminated with **** and the complete basis set is terminated with the $end keyword terminator. No blank lines can be incorporated within the general basis set input. Note that more than one contraction coefficient per line is one required for compound shells like SP. As with all Q-Chem input deck information, all input is case–insensitive.

Users are able to add comments to the input file outside keyword input sections, which will be ignored by the program. This can be useful as reminders to the user, or perhaps, when teaching another user to set up inputs. Comments can also be provided in a $comment block, which is actually redundant given that the entire input deck is copied to the output file.

$comment
   User comments - copied to output file
$end

$ecp
For each atom that will bear an ECP
      Chemical symbol for the atom
      ECP name; the $L$ value for the ECP; number of core electrons removed
      For each ECP component (in the order unprojected, ${\widehat{P}}_0$, ${\widehat{P}}_1$, , ${\widehat{P}}_{L-1}$
            The component name
            The number of Gaussians in the component
            For each Gaussian in the component
                  The power of $r$; the exponent; the contraction coefficient
****
$end

Note:  (1) All of the information in the $ecp block is case–insensitive. (2) The $L$ value may not exceed 4. That is, nothing beyond $G$ projectors is allowed. (3) The power of $r$ (which includes the Jacobian $r^{\mathrm{2}}$ factor) must be 0, 1 or 2.

$empirical_dispersion
    S6 S6_value
    D D_value
    C6 element_1 C6_value_for_element_1 element_2 C6_value_for_element_2
    VDW_RADII element_1 radii_for_element_1 element_2 radii_for_element_2
$end

Note:  This section is only for values that the user wants to change from the default values recommended by Grimme.

If the $external_charges keyword is present, Q-Chem scans for a set of external charges to be incorporated into a calculation. The format is shown below and consists of Cartesian coordinates and the value of the point charge, with one charge per line. The charge is in atomic units and the coordinates are in Ångstroms, unless bohrs are selected by setting the $rem keyword INPUT_BOHR to TRUE. The external charges are rotated with the molecule into the standard nuclear orientation and are specified in the following format:

$external_charges
   x-coord1  y-coord1  z-coord1  charge1
   x-coord2  y-coord2  z-coord2  charge2
   x-coord3  y-coord3  z-coord3  charge3
$end

In addition, the user can request to add a charged cage around the molecule (for so-called “charge stabilization” calculations) using the keyword ADD_CHARGED_CAGE. See Section 7.10.12 for details.

$intracule
       int_type 0 Compute $P(u)$ only 1 Compute $M(v)$ only 2 Compute $W(u,v)$ only 3 Compute $P(u)$, $M(v)$ and $W(u,v)$ 4 Compute $P(u)$ and $M(v)$ 5 Compute $P(u)$ and $W(u,v)$ 6 Compute $M(v)$ and $W(u,v)$ u_points Number of points, start, end. v_points Number of points, start, end. moments 0–4 Order of moments to be computed ($P(u)$ only). derivs 0–4 order of derivatives to be computed ($P(u)$ only). accuracy $n$ (${10}^{-n})$ specify accuracy of intracule interpolation table ($P(u)$ only). $end

Note that masses should be given in atomic units.

$isotopes
   number_extra_loops  tp_flag
      number_of_atoms  [temp pressure]
      atom_number1 mass1
      atom_number2 mass2
   ...
$end

A multipole field can be applied to the molecule under investigation by specifying the $multipole_field input section. Each line in this section consists of a single component of the applied field, in the following format:

$multipole_field
   field_component_1   value_1
   field_component_2   value_2
$end

Each field_component is stipulated using the Cartesian representation: $x$, $y$, or $z$ for dipole field components; $xx$, $xy$, etc. for quadrupole field components, $\mathrm{\dots }$. The value of each field component should be provided in atomic units. Contrary to the convention in many textbooks, within Q-Chem electric field lines are defined to run from negative charge to positive charge. ¹¹⁵² Shaik S. et al.
Chem. Soc. Rev.
(2018), 47, pp. 5125. Link ^{, 1151} Shaik S. et al.
J. Am. Chem. Soc.
(2020), 142, pp. 12551. Link

A dipole field component can also be specified using the indices of two atoms followed by the field magnitude. The field component will always be aligned parallel to the vector connecting atom1 and atom2, following the coordinates as the atoms move.

$multipole_field
   atom1 atom2 value
$end

Refer to Chapter 10 and the NBO manual for further information. Note that the NBO $rem variable must be set to ON to initiate the NBO package.

$nbo
   [ NBO options ]
$end

$occupied
   1  2  3  4 ...  nalpha
   1  2  3  4 ...  nbeta
$end

Note that units are in Ångstroms and degrees. Also see the summary in the next section of this Appendix.

$opt
CONSTRAINT
stre  atom1  atom2  value
...
bend  atom1  atom2  atom3  value
...
outp  atom1  atom2  atom3  atom4  value
...
tors  atom1  atom2  atom3  atom4  value
...
linc  atom1  atom2  atom3  atom4  value
...
linp  atom1  atom2  atom3  atom4  value
...
ENDCONSTRAINT

FIXED
atom   coordinate_reference
...
ENDFIXED

DUMMY
idum   type   list_length   defining_list
...
ENDDUMMY

CONNECT
atom   list_length   list
...
ENDCONNECT
$end

$svp
<KEYWORD>=<VALUE>, <KEYWORD>=<VALUE>,...
<KEYWORD>=<VALUE>
$end

For example, the section may look like this:

$svp
    RHOISO=0.001, DIELST=78.39, NPTLEB=110
$end

$svpirf
    <# point> <x point> <y point> <z point> <charge> <grid weight>
    <# point> <x normal> <y normal> <z normal>
$end

$plots
       One comment line
       Specification of the 3–D mesh of points on 3 lines:
             $N_x\mathit{\hspace{1em}}{x}_{\min }\mathit{\hspace{1em}}{x}_{\max }$
             $N_y\mathit{\hspace{1em}}{y}_{\min }\mathit{\hspace{1em}}{y}_{\max }$
             $N_z\mathit{\hspace{1em}}{z}_{\min }\mathit{\hspace{1em}}{z}_{\max }$
       A line with 4 integers indicating how many things to plot:
             $N_{\mathrm{MO}}\mathit{\hspace{1em}}{N}_{\mathrm{Rho}}\mathit{\hspace{1em}}{N}_{\mathrm{Trans}}\mathit{\hspace{1em}}{N}_{\mathrm{DA}}$
       An optional line with the integer list of MO’s to evaluate (only if $N_{\mathrm{MO}}>0$)
             MO(1)  MO(2) $\mathrm{\dots }$ MO($N_{\mathrm{MO}}$)
       An optional line with the integer list of densities to evaluate (only if $N_{\mathrm{Rho}}>0$)
             Rho(1)  Rho(2) $\mathrm{\dots }$ Rho($N_{\mathrm{Rho}}$)
       An optional line with the integer list of transition densities (only if $N_{\mathrm{Trans}}>0$)
             Trans(1)  Trans(2) $\mathrm{\dots }$ Trans($N_{\mathrm{Trans}}$)
       An optional line with states for detachment/attachment densities (if $N_{\mathrm{DA}}>0$)
             DA(1)  DA(2) $\mathrm{\dots }$ DA($N_{\mathrm{DA}}$)
$end

$localized_diabatization
       One comment line.
       One line with an an array of adiabatic states to mix together.
       $\mathrm{\dots }$
$end

Note:  We count adiabatic states such that the first excited state is , the fifth is , and so forth.

Note:  All radii are given in Ångstroms.

$van_der_waals
   1
   atomic_number   VdW_radius
$end

(alternative format)

$van_der_waals
   2
   sequential_atom_number   VdW_radius
$end

$xc_functional
   X  exchange_symbol  coefficient
   X  exchange_symbol  coefficient
   ...
   C  correlation_symbol  coefficient
   C  correlation_symbol  coefficient
   ...
   K  coefficient
$end