results were useful for the study of the phase behavior of hard particles and for the packing problem using Monte Carlo simulations. Although overlap criteria have been developed, analytic solutions for the distance of closest approach and the location of the point of contact have only recently become available. The details of the calculations are provided in Ref. The Fortran 90 subroutine is provided in Ref. The procedure consists of three steps: Transformation of the two tangent ellipses and , whose centers are joined by the vector , into a circle and an ellipse , whose centers are joined by the vector . The circle and the ellipse remain tangent after the transformation. Determination of the distance of closest approach of and analytically. It requires the appropriate solution of a quartic equation. The normal is calculated. Determination of the distance of closest approach and the location of the point of contact of and by the inverse transformations of the vectors and . Input: lengths of the semiaxes , unit vectors , along major axes of both ellipses, and unit vector joining the centers of the two ellipses. Output: distance between the centers when the ellipses and are externally tangent, and location of point of contact in terms of ,. Case of two ellipsoids Consider two ellipsoids, each with a given shape and orientation, whose centers are on a line with given direction. We wish to determine the distance between centers when the ellipsoids are in point contact externally. This distance of closest approach is a function of the shapes of the ellipsoids and their orientation. There is no analytic solution for this problem, since solving for the distance requires the solution of a sixth order polynomial equation. Here an algorithm is developed to determine this distance, based on the analytic results for the distance of closest approach of ellipses in 2D, which can be implemented numerically. Details are given in publications. Subroutines are provided in two formats: Fortran90 and C. The algorithm consists of three steps. Constructing a plane containing the line joining the centers of the two ellipsoids, and finding the equations of the ellipses formed by the intersection of this plane and the ellipsoids. Determining the distance of closest approach of the ellipses; that is the distance between the centers of the ellipses when they are in point contact externally. Rotating the plane until the distance of closest