Converting glm::lookat matrix to quaternion and back

/// <summary>
/// Converts the quaternion to it's matrix representation.
/// </summary>
/// <returns>A matrix representing the quaternion.</returns>
public Matrix ToMatrix()
{
    if (!this.Normalized)
        return this.Normalize().ToMatrix();

    double xx = X * X;
    double xy = X * Y;
    double xz = X * Z;
    double xw = X * W;

    double yy = Y * Y;
    double yz = Y * Z;
    double yw = Y * W;

    double zz = Z * Z;
    double zw = Z * W;

    return new Matrix(new double[,]
    {
        { 1 - 2 * (yy + zz), 2 * (xy - zw), 2 * (xz + yw), 0 },
        { 2 * (xy + zw), 1 - 2 * (xx + zz), 2 * (yz - xw), 0 },
        { 2 * (xz - yw), 2 * (yz + xw), 1 - 2 * (xx + yy), 0 },
        { 0, 0, 0, 1 }
    });
}

#include <glm/gtc/type_ptr.hpp> 
glMultMatrixf( glm::value_ptr( glm::mat4_cast(orientation) ) );

glMultMatrixf( &glm::mat4_cast(orientation)[0][0] );