Force field weaknesses

The performance is (as expected) very good. MMX provides relative (and absolute) stabilities with a MAD of only 1.2 kcal/mol, which is better than the estimates from the combined theoretical methods in Table 11.31. Considering that force field calculations require a factor of 10 less computer time for these systems than the ab initio methods combined in Table 11.31, this clearly shows that knowledge of the strengths and weakness of different theoretical tools is important in selecting a proper model for answering a given question. 

It is especially important to investigate the molecular structure of coordination compounds in the vapor phase because the relatively weak coordination interactions may be considerably influenced by intermolecular interactions in solutions and especially in crystals. It has been shown that the geometrical variations can be correlated with other properties of the molecular complexes ). In particular the structural changes in the F3B N(CH3)3 and CI3B N(CH3)3 molecules ) relative to the respective monomeric species unambiguously indicated boron trichloride to be a stronger acceptor than boron trifluoride. Data on the geometry and force field have also been correlated ). 

Chemists seeking to use computational chemistry to support experimental efforts now have three generd theoretical tools available to them force field or molecular mechanics models, ab initio molecular orbital (MO) models and semiempirical MO models (1). Each of these tools have strengths and weaknesses which must be evaluated to determine which is most appropriate for a given applications. 

In many actinide solids, as we shall see, the experimentally determined magnetic properties are explained well by assuming the permanent magnetic moment due to Hund s rules. The f-electrons are considered atomic, and their interaction with the environment is through crystal field forces or weak exchange forces with conduction electrons. Here, the magnetic properties are explained in the atomic limit. 

Group theory predicts that if parent CB is square, it should show four IR active fundamentals (of which one was a degenerate C—H stretch expected to be of very weak intensity). On the other hand, if parent CB were rectangular, it should show seven IR active fundamentals of which two are weak C—H stretches. Since only three (rather than five) bands were observed in the IR spectrum of CB below 2000 cm Krantz and Newton concluded that CB must be square. They supported this conclusion with a force-field calculation, based on a square geometry, that reproduced the observed frequencies, including those of C -d, to within a few wavenumbers. On the other hand, Chapman conjectured that the absence of evidence for two different vicinally dideuterated CR-CL2 (which would be expected for rectangular CB-[Pg.827]

All the quantities on the right hand side of the equation may be calculated using molecular mechanics force fields. However, it should be remembered that in many cases the binding of a drug to its target should be weak, because in most cases it has to be able to leave the target after it has activated that site. 

In this chapter we focus on atomistic predictions of thermophysical and mechanical properties of HMX crystals and liquid important to the development of reliable mesoscale equations of state. The outline of the remainder of the chapter is as follows In section 2 we describe briefly the philosophy and overall approach we have taken to force field development, including the results of quantum chemistry calculations for HMX and smaller model compounds that were used in the force field parameterization. The focus of section 3 is on the properties of liquid HMX, for which experimental data are completely lacking. Structural, thermal, and mechanical properties of the three pure crystal polymorphs of HMX are presented in section 4, where the results are compared to the available experimental data. At the ends of sections 3 and 4 we discuss briefly the importance of the various properties with mesoscale models of high explosives, with an emphasis on conditions relevant to weak shock initiation. We conclude in section 5, and provide our opinions (and justifications, based on our interactions with mesoscale modelers) regarding which HMX properties and phenomena should comprise the next targets for study via atomistic simulation. 

An important task for theory in the quest for experimental verification of N4 is to provide spectral characteristics that allow its detection. The early computational studies focused on the use of infrared (IR) spectroscopy for the detection process. Unfortunately, due to the high symmetry of N4(7)/) (1), the IR spectrum has only one line of weak intensity [37], Still, this single transition could be used for detection pending that isotopic labeling is employed. Lee and Martin has recently published a very accurate quartic force field of 1, which has allowed the prediction of both absolute frequencies and isotopic shifts that can directly be used for assignment of experimental spectra (see Table 1.) [16]. The force field was computed at the CCSD(T)/cc-pVQZ level with additional corrections for core-correlation effects. The IR-spectrum of N4(T>2 ) (3) consists of two lines, which both have very low intensities [37], To our knowledge, high level calculations of the vibrational frequencies have so far only been performed... 

On the non-platinated strand, the 5 -3 sequential C(5 -G ) H(2 )-G H(8) NOE cross-peak was very weak for PtR. This distance is quite short in all PtR models, 2.4-3.8 A. The weakness of this C(5 -G ) H(2 )-G H(8) NOE cross-peak could be due to a duplex structure very different from any model discussed here. Both the NP and LL PtC models have distances from 2.42 to 3.06 A, which would probably result in NOE cross-peaks more intense than those observed experimentally. In the PtW model, the corresponding C(5 -G ) H(2 )-A H(8) distance is 3.0 A, too short for the observed weak NOE cross-peak [73]. This distance also appears to be short in the PtL models [77], Thus, although experimental data indicate a large separation of the C(5 -G ) H(2 ) and G H(8) atoms, the force fields used in modeling calculations brings these two moieties close together. 

Points in support of Hypothesis III 1) Most models are unable to account for the weak C(5 -G ) H(2 )-G H(8) NOE cross-peak unless special restraints are included (Table 5). 2) The force field does not account explicitly for water, and a charged Pt moiety should alter the water structure. 3) The modeling assumes one correlation time, and the d(G pG ) moiety may lead to a greater divergence than normal in the correlation times of various protons in the duplex. 

This response is that of a permanent dipole that is partly oriented by a weak electrostatic field. "Weak" means that the energy put into orientation is much less than thermal energy the field gently perturbs otherwise random orientation. This response is slow x is so large that the contribution to forces from dipole orientation "counts" only in the n = 0 limit of low frequency. 

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