# LocalCoordinatesSite¶

class OpenMM::LocalCoordinatesSite

This is a VirtualSite that uses the locations of three other particles to compute a local coordinate system, then places the virtual site at a fixed location in that coordinate system. The origin of the coordinate system and the directions of its x and y axes are each specified as a weighted sum of the locations of the three particles:

origin = w1 r1 + w2 r2 + w3 r3 None

xdir = w1 r1 + w2 r2 + w3 r3 None

ydir = w1 r1 + w2 r2 + w3 r3 None

For the origin, the three weights must add to one. For example if (w1 , w2 , w3 ) = (1.0, 0.0, 0.0), the origin of the local coordinate system is at the location of particle 1. For xdir and ydir, the weights must add to zero. For excample, if (w1 , w2 , w3 ) = (-1.0, 0.5, 0.5), the x axis points from particle 1 toward the midpoint between particles 2 and 3.

The z direction is computed as zdir = xdir x ydir. To ensure the axes are all orthogonal, ydir is then recomputed as ydir = zdir x xdir. All three axis vectors are then normalized, and the virtual site location is set to

origin + x*xdir + y*ydir + z*zdir

Methods

 LocalCoordinatesSite() Create a new LocalCoordinatesSite() virtual site. getOriginWeights() Get the weight factors for the three particles when computing the origin location. getXWeights() Get the weight factors for the three particles when computing xdir. getYWeights() Get the weight factors for the three particles when computing ydir. getLocalPosition() Get the position of the virtual site in the local coordinate system.
LocalCoordinatesSite(int particle1, int particle2, int particle3, const Vec3 &originWeights, const Vec3 &xWeights, const Vec3 &yWeights, const Vec3 &localPosition)

Create a new LocalCoordinatesSite() virtual site.

• particle1 – the index of the first particle
• particle2 – the index of the second particle
• particle3 – the index of the third particle
• originWeights – the weight factors for the three particles when computing the origin location
• xWeights – the weight factors for the three particles when computing xdir
• yWeights – the weight factors for the three particles when computing ydir
• localPosition – the position of the virtual site in the local coordinate system
const Vec3 &getOriginWeights() const

Get the weight factors for the three particles when computing the origin location.

const Vec3 &getXWeights() const

Get the weight factors for the three particles when computing xdir.

const Vec3 &getYWeights() const

Get the weight factors for the three particles when computing ydir.

const Vec3 &getLocalPosition() const

Get the position of the virtual site in the local coordinate system.