F2Dock [CK+10,
BCS09,
CSB05]
is a rigidbody proteinprotein docking software developed
at
CVC (Computational Visualization Center), UT Austin,
in collaboration with
TSRI (The SCRIPPS Research Institute), California. The major contribution of
the original version of F2Dock [BCS09,
CSB05]
was the use of nonequispaced FFT to speedup docking calculations
while still remaining within a provable error bound. The current implementation
of F2Dock [CK+10]
focuses both on speed and accuracy, and includes a highly tunable solvation energy based
postprocessing tool called GBrerank
[MC+10]
for much improved reranking of the potential docking solutions generated by F2Dock.
F2Dock is used as a subroutine by F3Dock
[BCS08]
 a CVC software for flexible proteinprotein docking.
Docking Objective Functions/Filters and Postprocessing in F2Dock:
The current implementation of F2Dock [CK+10]
uses the following onthefly scoring functions and filters, and a solvation energy based postprocessing tool. Following the docking process, MolEnergy and TexMol can be used for further energetics computation and visualization.
 Exhaustive Search (FFT):
 Shape Complementarity: This is an improved version of the traditional
doubleskin layer based shape complementarity function. Unlike the traditional
model the receptor skin atoms do not touch the receptor surface, and the weight assigned
to a core atom is a function of its location in the molecule. The position and thickness
of the skin layers are carefully chosen for accuracy.
 Electrostatics: This is based on the approximate Coulombic
interaction function and designed to
reduce discretization errors on the grid.
 Hydrophobicity: Uses peratom hydrophonicity values to
compute hydrophobichydrophobic, hydrophobichydrophilic and hydrophilichydrophilic interactions.
 Hbond Correction: This function penalizes hydrogenbondlike
proximity of hydrogens of one molecule to the carbons and electronegative
atoms of the other which do not actually form hydrogen bonds.
 Filters (Multilevel Grid and/or Adaptive Spatial Decomposition):
 van der Waals (vdW) Filter: This function is used for onthefly
filtering of docking configurations with vdW potential above some userdefined
threshold. This static multilevel grid based implementation computes
the near interactions exactly and the far interactions approximately, and runs
up to 1500 times faster than the naïve implementation on a molecule
of typical size with less than 6% relative error. The approximation
scheme is described in [CB10].
 Clash Filter: Filters all docking poses with
the number of steric (atomatom) collisions above some userdefined threshold.
 Hydrophobicity Filter: Filters a docking configuration
if the ratio of buried hydrophilic area to buried hydrophobic area is outside some userdefined range.
 Interface Area Filter: Filters a docking pose
if the interface area is outside some userdefined range.
 Postprocessing:
 GBrerank (Solvation Energy): This tool reranks the docking configurations obtained from F2Dock
by computing an apporoximate change in solvation energy due to each configuration.
The polar part of the solvation energy (i.e., polarization energy) is approximated
in two phases using the surfacebased formulation of Generalized Born (GB) energy
[BZ09].
In the first phase the Born radius of each atom is approximated
using an octreebased fast summation algorithm. Another octreebased
fast approximation algorithm is used in the second phase that computes the polarization
energy from the approximated Born radii. Both approximation schemes
are described in [CB10].
The nonpolar part of the change in solvation energy
is approximated by computing an approximate interface area of the two
molecules using our fast linearspace Dynamic Packing Grid (DPG) data structure
described in [BCR09].
 MolEnergy: This package allows for more refined energy and binding affinity computations for selected results. This includes software for solving the PoissonBoltzmann equation and computing associated solvation energies [BCR09].
 Visualization:
 TexMol provides a graphical user interface and allows for the visualization of docking and energetics results.
Some Performance Figures (Boundbound Only; Unboundunbound Results are in [CK+10]):
The following figure shows how the performance of F2Dock improves as
various options described above are added one by one to the docking process.
The experiments were run on 60 rigidbody boundbound test cases from
ZDock benchmark 2.0. The test cases are listed
here
along with some relevant properties
(taken from [MC+10]).
F2Dock returned 2000 top solutions for
each complex, where a hit is any docking configuration
that lies within 5Å RMSD of the known solution. RMSD is
computed using all atoms of the moving molecule (i.e., ligand) that lie within
5Å of any atom of the static molecule (i.e., receptor) in the
known solution. Observe that in 80% of the cases F2Dock reports
a hit in the top 5.
The following figure compares the performance of the shape complementarity
function of F2Dock with that of the stateoftheart docking programs
ZDock 2.1
and DOT 2.0.
F2Dock performs significantly better than both.
F2Dock's shape complementarity function (with vdW filtering) runs much faster
than that of DOT 2.0, but is comparable to that of ZDock 2.1. The figure below
shows sequential running times of all three programs on a single core
of a 2.33 GHz dualcore Intel Xeon 5140. ZDock's shape complementarity
function requires a complextocomplex FFT in the forward direction and
a faster complextoreal FFT in the backward direction, while
F2Dock requires a complextocomplex FFT in both directions for shape complementarity.
But F2Dock speeds up FFT computation by taking advantage of
the sparsity of the input/output grids, restricting its search space
within a narrow band around the receptor, and using a high performance
priority queue in order to maintain the current pool of solutions. Thus F2Dock's
running time remains comparable to that of ZDock 2.1. However, ZDock 2.0.1
uses the high performance conv3D package
for computing 3D convolution, and as a result runs faster than F2Dock. F2Dock should
also be able to achieve similar speedups if conv3D is used.
The following figure compares the accuracy of F2Dock, ZDock 3.1 (/3.0.1)
and DOT 2.0 when electrostatics is also used with shape complementarity.
F2Dock still remains significantly more accurate than both ZDock
and DOT.
A detailed evaluation of the scoring functions of F2Dock
on both boundbound and unboundunbound test cases,
comparison of its performance with other similar docking software
(e.g., ZDock) as well as its performance on latest CAPRI targets can be found in
[KC+10].
A detailed comparative study of the reranking methods used
by F2Dock (i.e., GBrerank), ZDock (i.e., ZRank)
and DOT (i.e., ClusPro)
can be found in
[MC+10].
References
[BCR09] 
Chandrajit Bajaj, Rezaul A. Chowdhury, and Muhibur Rasheed.
A Dynamic Data Structure for Flexible Molecular Maintenance and Informatics.
Proceedings of the ACM Symposium on Solid and Physical Modeling (SPM 2009), San Francisco, California, pp. 259270, 2009.

[BCR11] 
Chandrajit Bajaj, ShunChuan Chen, and Alexander Rand.
An Efficient HigherOrder Fast Multipole Boundary Element Solution For PoissonBoltzmann Based Molecular Electrostatics.
To Appear in the SIAM Journal on Scientific Computing, 2011.

[BCS08] 
Chandrajit Bajaj, Rezaul A. Chowdhury, and Vinay Siddavanahalli.
F^{3}Dock: A Fast, Flexible and Fourier Based Approach to ProteinProtein Docking.
The University of Texas at Austin, ICES Report 0801, January 2008.

[BCS09] 
Chandrajit Bajaj, Rezaul A. Chowdhury, and Vinay Siddavanahalli.
F^{2}Dock: Fast Fourier ProteinProtein Docking.
IEEE/ACM Transactions on Computational Biology and Bioinformatics,8(1):4558, 2011.

[BZ09] 
Chandrajit Bajaj and Wenqi Zhao.
Fast Molecular Solvation Energetics and Force Computation.
SIAM Journal on Scientific Computing, 31(6):45244552, 2011.

[CB10] 
Rezaul A. Chowdhury and Chandrajit Bajaj.
Multilevel Grid Algorithms for Faster Molecular Energetics.
Proceedings of the ACM Symposium on Solid and Physical Modeling (SPM 2010), Haifa, Israel, September 13, 2010.

[CK+10] 
Rezaul A. Chowdhury, Donald Keidel, Maysam Moussalem, Arthur Olson, Michel Sanner, and Chandrajit Bajaj.
F^{2}Dock 2.0: Improved Fast Fourier ProteinProtein Docking.
Under Preparation, 2010.

[KC+10] 
Donald Keidel, Rezaul A. Chowdhury, Maysam Moussalem, Arthur Olson, Michel Sanner, and Chandrajit Bajaj.
F^{2}Dock 2.0 Scoring: An Evaluation.
Under Preparation, 2010.

[MC+10] 
Maysam Moussalem, Rezaul A. Chowdhury, Donald Keidel, Arthur Olson, Michel Sanner, and Chandrajit Bajaj.
Comparative Study of Three Reranking Methods for Fast Fourier TransformBased ProteinProtein Docking Programs.
Under Preparation, 2010.

