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15.2 Purpose of the Coordinate Manipulation Commands

All of these commands (except the unit cell and HBUILD commands) allow either the main coordinate set (default), or the comparison set (COMP keyword) to be modified. The other coordinate set is only changed by the SWAP command and the ORIEnt RMS command when the specified atoms are not centered about the origin.

Each of these commands may also operate on a subset of the full atom space (except the ORIEnt and unit cell commands). The selection specification should be at the end of the command. The default atom selection includes all atoms.

15.2.1 The INITIALIZE command

The INITIALIZE command returns the coordinate values of the specified atoms to their start up values (the parameter, ANUM which is currently 9999.0). The main use of this command is in connection with the BUILD BILD command, which may only find coordinates for atoms with the initial value.

15.2.2 The COPY command

The COPY command will copy the coordinate values into the specified set FROM the other coordinate set.

15.2.3 The SWAP command

The SWAP command will cause the coordinate values of the specified atoms to be swapped with the comparison set.

15.2.4 The AVERAGE command

The AVERAGE command will generate a new coordinate set at a point along the displacement vector between the present coordinate set and the other set. The FACTOR value determines the relative step along this vecor. Its default value is 0.5 (a true average). A FACTOR value of 1.0 is equivalent to the COPY command. Negative or greater than unit positive values are also allowed.

15.2.5 The TRANSLATE command

The TRANSLATE command will cause the coordinate values of the specified atoms to be translated. The translation step may be specified by either X, Y, and Z displacements, or by a distance along the specified vector. When no distance is specified, The XDIR, YDIR, and ZDIR values will be the step vector.

15.2.6 The ROTATE Command

The ROTATE command will cause the specified atoms to be rotated about the specified axis vector through the origin. The vector can specified either in terms of its Cartesian components, using the XDIR, YDIR, and ZDIR options, or by the vector between two atoms using the BYATOM option. The vector need not be normalized, but it must have a non zero length. The PHI value gives the amount of rotation about this axis in degrees (in the right handed sense). Only the atoms specified will be rotated.

15.2.7 The ORIENT Command

The ORIENT command will modify the coordinate values of all of the atoms. The select set of atoms is first centered about the origin, and then rotated to either align with the axis, or the other coordinate set. The RMS keyword will use the other coordinate set as a rotation reference. The MASS keyword cause a mass weighting to be done. This will align the specified atoms along their moments of inertia. When the RMS keyword is not used, then the structure is rotated so that its principle geometric axis coincides with the X-axis and the next largest coincides with the Y-axis. The SAVE option will result in the coordinate transformation being saved under the given name, and it can be used by XFORM option described below. This command is used for preparing a structure for graphics and viewing, for finding RMS differences, for model building, and in conjunction with the vibrational analysis.

15.2.8 The RMS Command

The RMS command will compute the Root Mean Square (RMS) or mass weighted RMS coordinate differences between the selected set of atoms just as they lie. This differences from the COOR ORIENT RMS command in that no coordinate modifications are made and no translation is done.

15.2.9 The RANDOM Command

The RANDOM command will randomize the selected set of atoms around their current coordinates. Two probability distributions are provided, normal (Gaussian) and uniform. For the normal distribution, the SD option specified the standard deviation of the distribution. For the uniform distribution, the SD option specifies the range of possible variation in both directions, i.e., new coordinates will have values anywhere between −SD and +SD. The default distribution is normal. The SEED option specifies the random number seed used to initialize the random number generator. It defaults to 1234.

15.2.10 The DIFF Command

The DIFF command will compute the differences between the main and comparison set (or the reverse) and store this difference in the coordinate displacement arrays, also referred to as the normal mode arrays (see Normal Modes). Each difference is scaled by the FACT keyword value. The differences are not appended onto current values. The differences may then be printed, etc...

15.2.11 The XFORM Command

The XFORM command will apply a stored coordinate transformation to a set of atoms. The coordinate transformation is specifed by name, and can be created by the ORIENT option above or by a analysis comparison command, see Comparisons. This command is useful when you want to move a set of atoms onto a position that is determined by another set of atoms.

15.2.12 The XINVERT Command

The XINVERT command will invert a stored coordinate transformation. The transformation is specified by the given name. No coordinates are affected by this command. For example, this command may be used to invert the transformation to orient a set of atoms, so that one set of atoms could be placed in the space occupied by another set. E.g.

     COOR ORIE SAVE A CLEAR ATOM 1 * *      ! Get transform for moving
                                            ! first segment to x axis.
     COOR XINV A                            ! Invert the transform
     COOR ORIE CLEAR ATOM 2 * *             ! Put second set on x axis
     COOR XFORM A CLEAR ATOM 2 * *          ! Move second set onto position
                                            ! of first segment.

15.2.13 The SCALE Command

The SCALE command will multiply the selected coordinates in the system by the given scale factor.

15.2.14 The CONVERT Command

The CONVERT command will convert from fractional coordinates to Cartesian coordinates. This command requires six real numbers; A, B, C, ALPHA, BETA, GAMMA; which are the unit cell parameters for the crystal. It assumes the current coordinates are in the fractional cell system, and it converts the selected atoms to Cartesian coordinates.

15.2.15 The EQUIVALENCE Command

The EQUIVALENCE command will generate symmetry related coordinates of the molecule. To use this command, you must first create a PSF which has multiple copies of the structure you wish to replicate by symmetry, and you must have read in the coordinates for the first assymetric unit. The EQUIVALENCE will first read a line containing just the number of transformations. Then it will read that number of transformations in the following format:

     ROTxx ROTxy ROTxz TRANSx
     ROTyx ROTyy ROTyz TRANSy
     ROTzx ROTzy ROTzz TRANSz

Each of the symbols above is parsed as a real number. The first transformation is expected to be the identity transformation and is not applied.

The coordinates are divided into blocks whose number equals the number of transformations. New coordinates for every block after the first are generated by multiplying the rotation matrix (ROTxx) by the coordinates (as a column matrix) of the first block and then adding the translation vector (TRANSx). Blank lines in this input are ignored.

The following input to CONGEN illustrates an example of generating coordinates for the alanines in a crystal of symmetry.

     open unit 11 name ala.crd form read
     read coor card unit 11
      1  0  0   0
      0  1  0   0
      0  0  1   0
     -1  0  0   0
      0 -1  0   0
      0  0  1   0.5
      0 -1  0   0
      1  0  0   0
      0  0  1   0.25
      0  1  0   0
     -1  0  0   0
      0  0  1   0.75

15.2.16 The Unit Cell Coordinate Manipulation Commands

The command COOR TRAN is a relic of the old code in CHARMM. There are no other keywords, but all of these commands require further input in formatted, fixed field form.

generates translated coordinates for molecules. See the routine TRAFAC in the file CORMAN.FLX for details.