ONIOM Keyword
Last Update: 06/04/2001

DESCRIPTION

This keyword requests a two- or three-layer ONIOM [338, 337, 340, 341, 336, 339, 418]. In this procedure, the molecular system being studied is divided into two or three layers which are treated with different model chemistries. The results are then automatically combined into the final predicted results.

The layers are conventionally known as the Low, Medium and High layers. By default, atoms are placed into the High layer. (From a certain point of view, a normal calculation can be viewed as a one-layer ONIOM.) Layer assignments are specified as part of the molecule specification (see below).

REQUIRED INPUT

The two or three desired model chemistries are specified as the options to the ONIOM keyword, in the order High, Medium, Low (the final one may obviously be omitted). The distinct models are separated by colons. For example, this route section specifies a three-layer ONIOM calculation, using Amber for the Low layer, AM1 for the Medium layer, and HF for the High layer:

# ONIOM(HF/6-31G(d):AM1:Amber)

Atom layer assignment is done as part of the molecule specification, via some additional parameters on each line according to the following syntax:

atom  coordinate-spec  layer [replace-atom [link-atom [scale-fac]]]

where atom and coordinate-spec represent the normal molecule specification input for the atom. Layer is a keyword indicating the layer assignment for the atom, one of High, Medium and Low. The other optional parameters specify how the atoms located at a layer boundary are to be treated. By default, atoms in a lower layer bound to an atom in a higher layer are replaced by hydrogen atoms during the higher-level part of the ONIOM calculation. You can use replace-atom to specify a different atom type with which to replace the atom. Similarly, link-atom can be used to specify which atom the current atom is to be bonded to during the higher-level calculation portion. scale-fac is the g-factor as defined in reference [418].

AVAILABILITY

Energies, gradients and frequencies. Note that if any of the specified models require numerical frequencies, then numerical frequencies will be computed for all models, even when analytic frequencies are available.

EXAMPLES

Here is a simple ONIOM input file:

# ONIOM(B3LYP/6-31G(d,p):AM1:Amber) Opt Test 

3-layer ONIOM optimization

0 1
C
O,1,B1
H,1,B2,2,A1
C,1,B3,2,A2,3,180.0,0  Medium  H
C,4,B4,1,A3,2,180.0,0  Low     H
H,4,B5,1,A4,5,D1,0     Medium
H,4,B5,1,A4,5,-D1,0    Medium
H,5,B6,4,A5,1,180.0,0  Low
H,5,B7,4,A6,8,D2,0     Low
H,5,B7,4,A6,8,-D2,0    Low

variable definitions

The High layer consists of the first three atoms (placed there by default). The other atoms are explicitly placed into the Medium and Low layers. Note that the Z-matrix specification must include the final 0 code indicating the Z-matrix format when ONIOM input is included.