`stre`- The
`stre`

(for**stre**tch) describes a distance between two atoms. It needs only two atomic indices to be given, the order of which is arbitrary. `invr`- The
`invr`

coordinate (for**inv**erse**r**) describes an inverse distance. The declaration is the same as for`stre`

, but in some cases (if you are far away from the minimum) the use of`invr`

may result in better convergence. `bend``bend`

describes a bond angle. It requires three atoms to be specified, of which the**third**one is the atom at the apex.`outp`- Out-of-plane angle:
`outp`*abcd*is the angle between bond*a*-*d*and plane*b*-*c*-*d*. `tors`- Dihedral angle:
`tors`*abcd*is the angle between the planes*a*-*b*-*c*and*b*-*c*-*d*. `linc`- This is a special coordinate type to
describe the bending of a near-linear system.
`linc`*abcd*describes the collinear bending of*a*-*b*-*c*(where the angle is defined as for`bend`

: the apex atom appears last)**in**the plane of*b*-*c*-*d*(see also below, command`linp`

). The system*b*-*c*-*d*has to be non-linear, of course. `linp`- This coordinate is similar to
`linc`

, but describes the bending of*a*-*b*-*c**perpendicular*to the plane*b*-*c*-*d*. These two types of coordinates are in most cases sufficient to describe the bending of near-linear systems. An example may help you to understand these two coordinate types. Consider ketene, H_{2}CCO, which contains a linear system of three atoms. Without symmetry, this molecule has 9 degrees of freedom. You could choose the four bond lengths, two CCH angles and the out-of-plane angle of the C-C bond out of the CHH-plane. But then two degrees of freedom still remain, which cannot be specified using these*normal*coordinate types. You can fix these by using`linc`

and`linp`

. The two coordinates`linc 1 3 2 4`

and`linp 1 3 2 4`

(where 1=oxygen, 2=carbon, 3=carbon, 4=hydrogen) would solve the problem. `comp`- The type
`comp`

describes a**comp**ound coordinate, i.e. a linear combination of (primitive) internal coordinates. This is often used to prevent strong coupling between (primitive) internal coordinates and to achieve better convergence of the geometry optimization. The use of linear combinations rather than primitive coordinates is especially recommended for rings and cages (see ref. [20]). Command`iaut`uses linear combinations in most cases.After you entered

`k comp`

*n*where*n*is the number of primitive internal coordinates to be combined, you will be asked to enter the type of the coordinate (`stre`

,`bend`

, ...). Then you will have to enter the weight (the coefficient of this primitive coordinate in the linear combination) and the atomic indices which define each coordinate. The definition of the primitive coordinates is the same as described above for the corresponding coordinate types. It is not possible to combine internal coordinates of different types. `ring`- This type helps you to define special
ring coordinates. You only have to enter
`k ring`

*n*where`n`is the ring size. Then you will be asked for the atomic indices of all atoms which constitute the ring and which must be entered in the same order as they appear in the ring. The maximum number of atoms in the ring is 69 (but in most cases the ring size will be limited by the maximum number of atoms which is allowed for`define`).

will bring you back to the internal coordinate
menu where you can see the new number of internal coordinates in the headline.

** Next:** Manipulating the Geometry
** Up:** Internal Coordinate Menu
** Previous:** Interactive Definition of Internal
** Contents**
** Index**
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