Geometry
is crucial in our efforts to comprehend the structures
and dynamics of biomolecules. For example, volume, surface area, and
integrated mean and Gaussian curvature of the union of balls representing
a molecule are used to quantify its interactions with the water surrounding
it in the morphometric implicit solvent models. The Alpha Shape theory
provides an accurate and reliable method for computing these geometric
measures. In this paper, we derive homogeneous formulas for the expressions
of these measures and their derivatives with respect to the atomic
coordinates, and we provide algorithms that implement them into a
new software package, AlphaMol. The only variables in these formulas
are the interatomic distances, making them insensitive to translations
and rotations. AlphaMol includes a sequential algorithm and a parallel
algorithm. In the parallel version, we partition the atoms of the
molecule of interest into 3D rectangular blocks, using a kd-tree algorithm. We then apply the sequential algorithm of AlphaMol
to each block, augmented by a buffer zone to account for atoms whose
ball representations may partially cover the block. The current parallel
version of AlphaMol leads to a 20-fold speed-up compared to an independent
serial implementation when using 32 processors. For instance, it takes
31 s to compute the geometric measures and derivatives of each atom
in a viral capsid with more than 26 million atoms on 32 Intel processors
running at 2.7 GHz. The presence of the buffer zones, however, leads
to redundant computations, which ultimately limit the impact of using
multiple processors. AlphaMol is available as an OpenSource software.