*LP3 is used to define the angular relationship of a vector to an sp3-plane.

**
Remarks**

In practice this command is most often used in studying the approach of a hydrogen atom to an sp3 oxygen atom and the consequent potential for hydrogen bonding.

Consider the following:

The format of the *LP3 command is:

DEFINE *LP3 <parnam1> <parnam2> i j k n

<parnam1> :

- The angle between the probe atom vector (k->n) and the plane bisecting
the
angle i k j, as illustrated in the Figure above. The range of values is
-90.0° <= parnam1 <= +90.0°. The plane bisecting the
angle i k j generates two hemispheres of parnam1 values. The hemisphere of
positive parnam1 values contains the vector k->i.
It should be noted that the definition of parnam1 is different from the polar angle usually found in spherical polar coordinate systems. Parnam1 starts from the plane bisecting the angle i k j instead of the normal to the plane as in the standard spherical polar coordinate system. This modification was made so that the angular parameters have more intuitive chemical meaning. A parnam1 value of 0.0 means the probe atom lies in the plane of the putative lone pairs of the sp3 atom.

<parnam2> :

- The angle between the projections of the vector k-> i and k-> n (the
probe atom vector) on to the plane bisecting the angle i j k. The range of
values is -180.0 <= parnam2 <= +180.0°. The plane i j k generates
two hemispheres of parnam2 values. The hemisphere of positive parnam2 values
is identified by the cross product of k->i and k->j (the parnam2 angle
identified in the Figure thus has a positive value).
<parnam2> is the same as the azimuth angle in the standard spherical polar coordinate system. A value of +/-125.25 means the probe atom is collinear with an sp3 - lone pair direction.

i j k n :

- Atom numbers, centroids and/or dummy points.
i j k specify an sp3 plane with origin k, and n is the probe atom for calculating angular parameters. i and j define the positive hemispheres for parnam1 and parnam2. The order in which i j k n are given is CRITICAL to obtain the correct results.

**
Example**

*Select *LP3*Select or type items for *LP3. If necessary reselect *LP3 to end input. Select CANCEL to cancel command.

*Select atom number 2***2**appears in the dialogue areaA red + appears at atom 2 in the fragment.

*Select in turn atoms 4 3 1*As each atom is selected a red + appears at the atom in the fragment.

After the selection of the fourth atom these red crosses disappear.

- In the dialogue area we now have:
DEFINE, *LP3, Input *LP3 parameter name no. 1

*Type THETA <RETURN>*- In the dialogue area we now have:
DEFINE, *LP3, THETA, Input *LP3 parameter name no. 2

*Type PHI <RETURN>*- In the summary area we now have:
*LP3 THETA PHI 2 4 3 1

This line is displayed in green to indicate that it is a DEFINE command.

In the dialogue area we now have the "home" prompt:

Search for ALL crystal fragments OVERLAP of crystal fragments permitted Select a menu command or TO-BUILD to return to the BUILD menu.

The resultant command in the instruction document is:

DEFINE *LP3 THETA PHI 2 4 3 1

The order in which the four atoms, centroids and/or dummy points i j k n are specified is important.

The non-bonded vector must be between the atoms, centroids and/or dummy points k and n.

In a single search test there can be a maximum of 10 *LP3 instructions.

**
Related Command**

DEFINE *LP2

Volume 2 Chapter 3 3D-Constrain Sub-menu *MASK.