Keywords REACTANT and PRODUCT

These two keywords must appear together (cannot be used separately, but can be used in either order). They define the reactant and product structures for a saddle-point (see 4.6.2) interpolation of a transition state structure.
Options:
CARTESIAN / ZMATRIX / MIXED

	CARTESIAN		The molecular structure is given in Cartesian coordinates.
	ZMATRIX		The molecular structure is defined by a Z-matrix. Equivalent to the option ZMATRIX is the option Z-MATRIX.
	MIXED		The first atoms of the molecular structure are defined by Cartesian coordinates and the following ones are defined by a Z-matrix.

ANGSTROM / BOHR

	ANGSTROM		Coordinates or bond distances are given in Ångströms.
	BOHR		Coordinates or bond distances are given in atomic units.

Description:
The reactant and product geometries for a saddle-point interpolation are read in the keyword bodies of REACTANT and PRODUCT, respectively. Both structures must be local minima on the potential energy surface. If the molecular structures are defined by a Z-matrix, two different VARIABLE definitions are allowed but CONSTANTS must be the same for both REACTANT and PRODUCT . In the following input example, the first variable definition is assigned to the product whereas the second one is assigned to the reactant.

 PRODUCT Z-MATRIX ANGSTROM
 C                                                         6    12.011000
 C       1  R1                                             6    12.011000
 O       1  R2         2  A1                        RAD    8    15.999400
 H       3  R3         1  A2         2  D1          RAD    1     1.007940
 H       2  R4         1  A3         3  D2          RAD    1     1.007940
 H       1  R5         2  A4         3  D3          RAD    1     1.007940
 H       2  R6         1  A5         3  D4          RAD    1     1.007940
 #
 VARIABLES
 R1          1.33753412
 R2          1.35406571
 R3           .98316245
 R4          1.09714328
 R5          1.10195029
 R6          1.10140201
 A1          126.727334
 A2          108.484045
 A3          120.171938
 A4          122.350600
 A5          122.089435
 D1             .026953
 D2          180.007587
 D3         -180.000000
 D4             .000000
 #
 #
 #
 REACTANT Z-MATRIX ANGSTROM
 C                                                         6    12.011000
 O       1  R1                                             8    15.999400
 C       1  R2         2  A1                        RAD    6    12.011000
 H       3  R3         1  A2         2  D1          RAD    1     1.007940
 H       3  R4         1  A3         2  D2          RAD    1     1.007940
 H       3  R5         1  A4         2  D3          RAD    1     1.007940
 H       1  R6         2  A5         3  D4          RAD    1     1.007940
 #
 VARIABLES
 R1          1.21898657
 R2          1.48370403
 R3          1.10453256
 R4          1.11075723
 R5          1.11077688
 R6          1.13169337
 A1          124.467029
 A2          111.060189
 A3          109.154845
 A4          109.123964
 A5          120.433172
 D1            -.070961
 D2         -122.316147
 D3          122.144371
 D4          179.992103

Constant (frozen) structure parameters must be common to both reactants and products. Thus, only one set of constant definitions is permitted. The following input is used for a saddle-point interpolation in which the positions of C1 and C2 remain unchanged.

 PRODUCT
 C1        .000000       .000000       .000000
 C2        .000000       .000000      1.508857
 H        1.028689       .000000      -.410316
 H       -1.015045      -.024889      1.940572
 H        -.529072      -.891070      -.389824
 H        -.517448       .901443      -.385027
 H         .544687       .857413      1.954166
 H         .676484      -.872804      1.873463
 SI       -.129351     -2.427077      2.452048
 H       -1.252212     -2.428853      1.388199
 H       -1.024260     -1.779782      3.537837
 #
 #
 #
 REACTANT
 C1        .000000       .000000       .000000
 C2        .000000       .000000      1.508857
 H        1.035894       .000000      -.397290
 SI        .882110     -1.504909      2.206066
 H        -.504846      -.904544      -.397302
 H        -.520431       .886537      -.415328
 H         .492408       .912128      1.916629
 H       -1.036466       .015343      1.916608
 H         .883975     -1.510440      3.722866
 H         .196804     -2.766324      1.714760
 H        2.318323     -1.522790      1.716988
 #
 CONSTANTS
 C XYZ

Please note that the Cartesian coordinates of these two atoms have to be the same in the PRODUCT and REACTANT definitions. In fact, these two structures were optimized with the same constraint. Examples and on page and in the tutorial, show typical inputs for saddle point interpolations.