COMPASS force field

A force field provides a mathematical description of the potential energy as a function of the atomistic configuration of the polymer. The amorphous packing of the PEEK-WC polymer have been constructed using the force field COMPASS (condensed-phase optimized molecular potentials for atomistic... 

Another important component of the COMPASS force-field development is the systematic fit to various types of compounds. First, alkane parameters are fit, and then held fixed while alkene parameters are derived. Each functional group is added by fitting to larger arrays of compounds, but the parameters derived in the previous level are not changed, so a self-consistent force field is created that describes a wide variety of functional groups. 

COMPASS has been used to model a number of condensed-phase organic systems.57-59 Hence, we chose COMPASS as a likely candidate for modeling fulvic and humic acids however, we caution the reader that classical mechanical force fields may be accurate for one system and not for another. The force field must be tested for each new application before the results can be considered reliable. 

FIGURE 5.8 Model of SFA with a -4 charge in a periodic box of 1493 H20 molecules and charge balanced with four Na+ ions after 50 ps of MD simulation with the COMPASS force field in Cerius2 (Accelrys, Inc.). Results of deprotonation and solvation are similar in these three-dimensional periodic simulations to the nonperiodic energy minimization in Figure 5.7(c). 

Sun, H., COMPASS An ab initio force-field optimized for condensed-phase applications—Overview with details on alkane and benzene compounds, J. Phys. Chem., 102, 7338, 1998. 

The X-ray structures of quartz and calcite were obtained from the FIZ Karlsruhe ICSD database [76]. The chosen model structures were constructed in MATERIALS STUDIO [77] under periodic boundary conditions and were then energy-minimized using the program DISCOVER with COMPASS force field until a maximum derivative better than 0.01 kcal mor A was achieved. 

Fa. Accelrys, Materials Studio with DISCOVER 3.01 and COMPASS - force field version 3.0. 

Table 3-4. Relative energies [per molecule, kcal mol ] of melt and sublimed phases 5m and 5s. Ujj(( were calculated using COMPASS force field in Cerius and E onf was computed in Gaussian 03 at the DFT, B3LYP/6-3IG level...

Bunte, S.W., and H. Sun. 2000. Molecular modeling of energetic materials The parameterization and validation of nitrate esters in the COMPASS force field. / Phys Chem B 104 2477. 

Cerius, COMPASS force field, Accelrys Inc., San Diego, USA, www.accelrys. com. 

The OPLS Q conformations were correctly packed to reproduce three out of four forms forms I, VI, and XLI. In fact, each of the predicted forms was ranked among the top few solutions from the 500 generated polymorphs as ranked by COMPASS force field. The predicted crystal forms displayed a good overlay with the experimental aystal structures (see Fig. 4.9), with RMSDs of0.66,0.54, aud 0.47 A for the overlay of 10 molecules in the crystal. In contrast, the CSPs based on conformations generated by the MMFFs and OPLS 2005 force fields failed to reproduce the crystal structures of forms I and XLI within any of the top 500 solutions. Only form VI was reproduced starting from the MMFFs conformation, resulting in an RMSD of 0.54 A for the overlay of 10 molecules. 

A crystal morphology simulation for ibuprofen was performed with Accelrys s Materials Studio software suite, utilizing the morphology module. The crystal structure was originally obtained from the CSD and then fully optimized using the COMPASS force field—a priori to morphological calculations. As shown in Figure 6.12, the unrelaxed vacuum slabs of crystal faces with thickness of 25 A were constructed from the optimized crystal stracture. 

Universal. The parameters in the Universal force field (UFF) [14-16] are calculated using general rules based only upon the element, its hybridization, and its connectivity. For this reason, the UFF has broad applicability but is inherently less accurate than extensively parameterized force fields such as COMPASS. Bond-stretching terms in the UFF are either harmonic or Morse functions. The anglebending and torsion terms are described by a small cosine Fourier expansion. For nonbonded terms, the LJ 6-12 potential and Coulombic terms are used for steric and electrostatic terms, respectively. 

COMPASS. COMPASS is an example of a Class II force field parameterized by using an analytic representatiOTi of the ab initio (e.g., HF/6-31G ) potential energy surface. The functional form of the COMPASS force field is the same as CFF93 and includes an out-of-plane potential term (angle X), a U 6-9 potential as well as nonharmonic terms for bond stretching and angle bending, a Fourier cosine series for torsion, and a number of cross-coupled terms for the bonded interactions. The form of the COMPASS force field described in detail by Sun [12] is... 

Class n force fields make extensive use of both anharmo-nic and cross-coupling terms to adequately represent the ab initio potential energy surface (PES). These include the original consistent force field (CFF) that developed out of CVFF (Section 4.2.2) and subsequent variations, the most recent being the COMPASS force field. 

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