Pseudo Keyword
Last Update: 12/31/2000

This keyword requests that a model potential be substituted for the core electrons. The Cards option is by far its most used mode. Gaussian 98 supports a new effective core potential (ECP) input format (similar to that used by ExtraBasis) which is described below. When reading-in pseudopotentials, do not give them the same names as any internally-stored pseudopotentials: CEP, CHF, LANL1, LANL2, LP-31, SDD and SHC.

Read
Read pseudo-potential data from the input stream. Input is described in the next subsection below. Cards is a synonym for Read.

Old
Read pseudo-potential data using the old format (used by Gaussian 92 and earlier versions).

CHF
Requests the Coreless Hartree-Fock potentials. This option is normally used with the LP-31G basis sets.

SHC
Requests the SHC potentials.

LANL1
Requests the LANL1 potentials.

LANL2
Requests the LANL2 potentials.

Effective Core Potential operators are sums of products of polynomial radial functions, Gaussian radial functions and angular momentum projection operators. ECP input therefore specifies which potential to use on each atomic center, and then includes a collection of triplets of (coefficient, power of R, exponent) for each potential for each term in each angular momentum of the ECP. Since only the first few angular momentum components have different terms, the potential is expressed as (1) terms for the general case, typically d or f and higher projection, and (2) the extra terms for each special angular momentum. Thus for an LP-31G potential, which includes special s and p projected terms, the input includes the general (d and higher) term, the s-d term (i.e., what to add to the general term to make the s component) and the p-d term.

ECP input has changed somewhat with Gaussian 98 to allow ECPs in internally-stored basis sets to be referenced.

All ECP input is free-format. Each block is introduced by a line containing the center numbers (from the molecule specification) and/or atomic symbols, specifying the atoms and/or atoms types to which it applies (just as for general basis set input--see the discussion of the Gen keyword). The list ends with a value of 0.

The pseudo-potential for those centers/atoms follows:

Name,Max,ICore Name of the potential, maximum angular momentum of the potential (i.e., 2 if there are special s and p projections, 3 if there are s, p, and d projections), and number of core electrons replaced by the potential. If Name matches the name of a previous potential, that potential is reused and no further input other than the terminator line (see below) is required.

For each component (I=1 to Max) of the current potential, a group of terms is read, containing the following information:

Title -- A description of the block, not otherwise used.

NTerm -- Number of terms in the block.

NPower,Expon,Coef -- Power of R, exponent, and coefficient for each of the NTerm terms. NPower includes the R² Jacobian factor.

An example of an input file which includes a nonstandard ECP with its associated basis set is given below.

Gaussian 98 adds flexibility to ECP input by allowing it to include pre-defined basis sets names. An ECP definition may be replaced by a line containing the standard keyword for a pre-defined basis set. In this case, the ECPs within the specified basis set corresponding to the specified atom type(s) will be used for that atom (see the examples).

In Pseudo input, keywords for these ECP's are of the form ECPnXY where n is the number of core electrons which are replaced by the pseudopotential and X denotes the reference system used for generating the pseudopotential (S for a single-valence-electron ion or M for a neutral atom).

Y specifies the theoretical level of the reference data: HF for Hartree-Fock, WB for Wood-Boring quasi-relativistic and DF for Dirac-Fock relativistic. For one- or two-valence electron atoms SDF is a good choice; otherwise MWB or MDF is recommended (although for small atoms or for the consideration of relativistic effects, the corresponding SHF and MHF pseudopotentials may be useful).

Energies through f functions only, and gradients through d functions only.

ChkBasis, ExtraBasis, Gen

Specifying an ECP. This input file runs an RHF/LP-31G calculation on hydrogen peroxide, with the basis set and ECP data read from the input file:

# HF/Gen Pseudo=Read Test

Hydrogen peroxide

0,1
O
H,1,R2
O,1,R3,2,A3
H,3,R2,1,A3,2,180.,0

R2=0.96
R3=1.48
A3=109.47

General basis set input
****


O 0           ECPs for the oxygen atoms.
OLP 2 2       ECP name=OLP, applies to d & higher, replaces 2 electrons.
D component   Description for the general terms.
3             Number of terms to follow.
1 80.0000000  -1.60000000
1 30.0000000  -0.40000000
2  1.0953760  -0.06623814
S-D projection Corrections for projected terms (lowest angular momentum).
3
0  0.9212952   0.39552179
0 28.6481971   2.51654843
2  9.3033500  17.04478500
P-D            Corrections for projected terms (highest  angular momentum).
2
2 52.3427019  27.97790770
2 30.7220233 -16.49630500
****           End of the ECP block for oxygen.

The basis set data follows the molecule specification section. The first line of the ECP data requests that a potential be read in (type 7) for atoms number 1 and 3 (the oxygen atoms) and that no potential is to be used for atoms 2 and 4 (the hydrogen atoms).

The second line of ECP data begins the input for the first center requiring a read-in potential, in this case oxygen atom 1. The potential on this center is named OLP, it is general term and applies to angular momentum 2 (D) and higher, and the potential replaces two electrons. Next comes a title for the general term, the number of components of that term, and each of the components, followed by the corrections for the projected terms, lowest angular momentum first.

Finally, the next potential, for center 3 in this case, consists of a single line. It uses the same name as a previous potential (that of center 1) and so the information already read in is reused. Note that the maximum angular moment and number of core electrons must still be specified, even though they will generally be the same for all uses of a given potential.

Using Standard Basis Set Keywords to Specify ECPs. The following input file illustrates the use of the simplified ECP input format:

# Becke3LYP/Gen Pseudo=Read Opt Test

HF/6-31G(*) Opt of Cr(CO)6

0 1

Cr 0.0  0.0  0.0
molecule specification continues ...

C O 0
6-31G(d)
****
Cr 0
LANL2DZ
****

Cr 0           ECP for chromium atom.
LANL2DZ        Use the ECP in this basis set.

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