Gaussian 03 Online Manual
This keyword requests a two- or three-layer ONIOM [153,154,155,156,157,158,159]. 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, any conventional calculation can be viewed as a one-layer ONIOM.) Layer assignments are specified as part of the molecule specification (see below).
For ONIOM(MO:MM) jobs, the ONIOM optimization procedure is enhanced in Gaussian 03 to use microiterations  and an optional quadratic coupled algorithm . The latter takes into account the coupling between atoms using internal coordinates (typically, those in the model system) and those in Cartesian coordinates (typically, the atoms only in the MM layer) in order to produce more accurate steps (the latter can be requested with Opt=QuadMacro).
ONIOM(MO:MM) calculations can take advantage of electronic embedding. Electronic embedding incorporates the partial charges of the MM region into the quantum mechanical Hamiltonian. This technique provides a better description of the electrostatic interaction between the QM and MM regions and allows the QM wavefunction to be polarized.
ONIOM calculations can also use an external program for the calculations for one or more layers. See the External keyword for details.
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 UFF for the Low layer, AM1 for the Medium layer, and HF for the High layer:
Atom layer assignment is done as part of the molecule specification, via additional parameters on each line according to the following syntax:
atom coordinate-spec layer [link-atom [bonded-to [scale-fac1 [scale-fac2 [scale-fac3]]]]]
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. You use link-atom to specify the atom with which to replace the current atom (it can include atom type and partial charge and other parameters). Link atoms are necessary when covalent bonding exists between atoms in different layers in order to saturate the (otherwise) dangling bonds.
Note: All link atoms must be specified by the user. Gaussian 03 does not define them automatically or provide any defaults.
The bonded-to parameter specifies which atom the current atom is to be bonded to during the higher-level calculation portion. If it is omitted, Gaussian will attempt to identify it automatically.
In general, Gaussian 03 determines bond distances between atoms and their link atoms by scaling the original bond distance (i.e., in the real system), using scaling factors which the program determines automatically. However, you can also specify these scale factors explicitly. For a two-layer calculation, the scale factors specify the link atom bond distance in the model system when calculated at the low and high levels (respectively). For a three-layer ONIOM, up to three scale factors may be specified (in the order low, medium, high). All of these scale factors correspond to the g-factor parameter as defined in reference , extended to allow separate values for each ONIOM calculation level.
For a two-layer ONIOM, if only one parameter is specified, then both scale factors will use that value. For a three-layer ONIOM, if only one parameter is specified, then all three scale factors will use that value; if only two parameters are specified, then the third scale factor will use the second value.
If a scale parameter is explicitly set to 0.0, then the program will determine the corresponding scale factor in the normal way. Thus, if you want to change only the second scale factor (model system calculated at the medium level), then you must explicitly set the first scale factor to 0.0. In this case, for a three-layer ONIOM, the third scale factor will have the same value as the second parameter unless it is explicitly assigned a non-zero value (i.e., in this second context, 0.0 has the same meaning as an omitted value).
PER-LAYER CHARGE AND SPIN MULTIPLICITY
Multiple charge and spin multiplicity pairs may also be specified for ONIOM calculations. For two-layer ONIOM jobs, the format for this input line is:
chrgreal-low spinreal-low [chrgmodel-high spinmodel-high [chrgmodel-low spinmodel-low [chrgreal-high spinreal-high]]]
where the subscript indicates the calculation for which the values will be used. The fourth pair applies only to ONIOM=SValue calculations. When only a single value pair is specified, all levels will use those values. If two pairs of values are included, then the third pair defaults to the same values as in the second pair. If the final pair is omitted for an S-value job, it defaults to the values for the real system at the low level.
Values and defaults for three-layer ONIOM calculations follow an analogous pattern (in the subscripts below, the first character is one of: Real, Int=Intermediate system, and Mod=Model system, and the second character is one of: H, M and L for the High, Medium and Low levels):
cRealL sRealL [cIntM sIntM [cIntL sIntL [cModH sModH [cModM sModM [cModL sModL]]]]]
For 3-layer ONIOM=SValue calculations, up to three additional pairs may be specified:
... cIntH sIntH [cRealM sRealM [cRealH sRealH]]
Defaults for missing charge/spin multiplicity pairs are taken from the next highest calculation level and/or system size. Thus, when only a subset of the six or nine pairs are specified, the charge and spin multiplicity items default according to the following scheme, where the number in each cell indicates which pair of values applies for that calculation in the corresponding :circumstances:
Specifies scaling parameters for MM charges during electronic embedding in the QM calculations. The integers are multiplied by 0.2 to obtain the actual scale factors. Atoms bonded to the inner layers use a scale factor of 0.2n, those two bonds away use 0.2m, and so on. However, the values of i through n must be monotonically decreasing, and the largest value among them is used for all parameters to its left. Thus, 555500, 123500 and 500 are all equivalent. The default value is 500 (i.e., 555500). ScaleCharge implies EmbedCharge.
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.
ONIOM can also perform CIS and TD calculations for one or more layers. The Gen, Pseudo=Read, ChkBas, Sparse and NoFMM keywords may also be specified for relevant models. Density fitting sets may also be used when applicable, and they are specified in the usual manner (see the examples).
NMR calculations may be performed with the ONIOM model.
Molecule Specifications for ONIOM Jobs. Here is a simple ONIOM input file:
# ONIOM(B3LYP/6-31G(d,p):AM1:UFF) 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 M H C,4,B4,1,A3,2,180.0,0 L H H,4,B5,1,A4,5,D1,0 M H,4,B5,1,A4,5,-D1,0 M H,5,B6,4,A5,1,180.0,0 L H,5,B7,4,A6,8,D2,0 L H,5,B7,4,A6,8,-D2,0 L
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.
Here is an input file for a two-layer ONIOM calculation using a DFT method for the high layer and Amber for the low layer. The molecule specification includes atom types (which are optional with UFF but required by Amber). Note that atom types are used for both the main atom specifications and the link atoms:
# ONIOM(B3LYP/6-31G(d):Amber) Geom=Connectivity 2 layer ONIOM job 0 1 0 1 0 1 Charge/spin for entire molecule (real system), model system-high level & model-low C-CA--0.25 0 -4.703834 -1.841116 -0.779093 L C-CA--0.25 0 -3.331033 -1.841116 -0.779093 L H-HA-0.1 3 C-CA--0.25 0 -2.609095 -0.615995 -0.779093 H C-CA--0.25 0 -3.326965 0.607871 -0.778723 H C-CA--0.25 0 -4.748381 0.578498 -0.778569 H C-CA--0.25 0 -5.419886 -0.619477 -0.778859 L H-HC-0.1 5 H-HA-0.1 0 -0.640022 -1.540960 -0.779336 L H-HA-0.1 0 -5.264565 -2.787462 -0.779173 L H-HA-0.1 0 -2.766244 -2.785438 -0.779321 L C-CA--0.25 0 -1.187368 -0.586452 -0.779356 L H-HA-0.1 3 C-CA--0.25 0 -2.604215 1.832597 -0.778608 H H-HA-0.1 0 -5.295622 1.532954 -0.778487 L H-HA-0.1 5 H-HA-0.1 0 -6.519523 -0.645844 -0.778757 L C-CA--0.25 0 -1.231354 1.832665 -0.778881 L H-HC-0.1 11 C-CA--0.25 0 -0.515342 0.610773 -0.779340 L H-HA-0.1 0 -3.168671 2.777138 -0.778348 L H-HA-0.1 11 H-HA-0.1 0 -0.670662 2.778996 -0.779059 L H-HA-0.1 0 0.584286 0.637238 -0.779522 L 1 2 1.5 6 1.5 8 1.0 2 3 1.5 9 1.0 3 4 1.5 10 1.5 4 5 1.5 11 1.5 5 6 1.5 12 1.0 6 13 1.0 7 10 1.0 8 9 10 15 1.5 11 14 1.5 16 1.0 12 13 14 15 1.5 17 1.0 15 18 1.0 16 17 18
This input file was created by GaussView. Note that it contains connectivity information for use with Geom=Connect. This job also illustrates the use of multiple charge and spin multiplicity values for ONIOM jobs. This example should be used for illustrative purposes only. We thank Prof. K Nishimoto for pointing out several scientific problems with running this calculation.
A Complex ONIOM Route. Here is an example of a complex ONIOM route section:
# ONIOM(BLYP/6-31G(d)/Auto TD=(NStates=8):UFF)
This example uses density fitting for the DFT high layer time-dependent excited states calculation.
Freezing Atoms During ONIOM Optimizations. Just as for non-ONIOM geometry optimizations, ONIOM optimizations can take advantage of the optional second field within molecule specifications. This field defaults to 0 if omitted and the atom position is included in the geometry optimization. If the value of the second field is set to -1, then the corresponding atom is frozen during geometry optimizations, as in this example:
C -1 0.0 0.0 0.0 H 0 0.0 0.0 0.9
For ONIOM jobs only, if the field is set to a negative value other than -1, it is treated as part of a rigid fragment during the optimization: all atoms with the same value (< -1) move only as a rigid block. Only MM atoms can be members of such rigid fragments.
S-Value Test. Here is some output from the ONIOM=SValue option:
S-Values (between gridpoints) and energies: high 4 -39.322207 7 -39.305712 9 -114.479426 -153.801632 -193.107344 med 2 -39.118688 5 -39.106289 8 -114.041481 -153.160170 -192.266459 low 1 -38.588420 3 -38.577651 6 -112.341899 -150.930320 -189.507971 model mid real
The integers are the gridpoints, and under each one is the energy value. Horizontally between the grid points are the S-values. These are the S-values obtained with the absolute energies. However, be aware that when applying the S-value test, relative energies and S-values need to be used (see reference ).