Force COMPASS

Per-Olov Lowdin established both the Quantum Chemistry Group at Uppsala and the Quantum Theory Project at Gainesville through his personal and scientific stature and he was unquestionably the Director of both operations. Several prominent scientists were invited during the first years to be Associate Directors and to maintain the spirit when "the Force" was absent. Yngve Ohrn was invited to join the faculty at Florida as Associate Professor in 1966 and he chose to become a member of the Quantum Theory Project. He assumed the Associate Directorship of the Quantum Theory Project in 1967 and proved quickly to have the qualities that allowed him to deal effectively with the university administration at all levels as well as with issues within the rather diverse and extensive body of scientists and students of the Project. Fairness, compassion, and persistence are words that come to my mind when I characterize Yngve s leadership. 

Many junior high and high schools around the country require their students to spend a certain number of hours doing volunteer work or community service. Some people believe that this is an excellent idea that promotes good citizenship and cultivates compassion. Others feel that forced volunteerism is not actually volunteerism at all. 

Imagine for a moment that we were able to switch off the earth s magnetic field. One effect would be to make navigation much harder since all compasses would be useless. They would be free to point in whatever direction they wanted to and, if we turned the needle round, it would simply stay where we left it. However, as soon as we switched the magnetic field back on, they would all point north—their lowest energy state. Now if we wanted to force a needle to point south we would have to use up energy... 

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 aim hae is to take a microscopic view of an ion inside a solvent. The central consideration is that ions orient dipoles. The spherically symmetrical electric field of the ion may tear water dipoles out of the water lattice and make them point (like compass needles oriented toward a magnetic pole) with the appropriate charged end toward the central ion. Hence, viewing the ion as a point charge and the solvent molecules as electric dipoles, one obtains apicture of ion-dipole forces as the principal source of ion-solvent interactions. 

Slowly I become aware of self again. I feel nausea and pain and disgust and recognize these as part of the creative force, unique qualities born of the experiential, to be realized fully. It is necessary to feel the nausea of nausea, the horror of horror, the disgust of disgust, all in their pure forms. Out of this same spirit come beauty, love, and compassion. 

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. 

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