We have applied the fully variational molecular orbital (FVMO) method to systematically improve the values of energy-components, that is, the kinetic energy of electron and potential energy, as with that of total energy. The FVMO method enables us to describe accurate values of energy-components, since the quantum-mechanical Virial and Hellmann-Feynman theorems are completely satisfied due to the optimization of the exponents and centers in Gaussian-type functions (GTFs) basis sets. In the calculation of H2 molecule, we have found that the energy components and total energy with FVMO method using only [6s] GTFs are actually improved more than those with the conventional MO using a much larger basis set including the multiple polarization functions of [6s3p2d1f]. Additionally, drastic improvement of virial ratios for several hydride molecules with the FVMO method is demonstrated in the current work. The energy-component analysis using the FVMO method would be a powerful tool to elucidate various chemical problems.
To elucidate the adequate molecular exponents for sp3-, sp2-, and sp-hybrid characters, we have performed the optimization for both geometry and Gaussian-type function (GTF) exponents using various hydrocarbon molecules. We have found that the scale factor 1.2 is significant for the p-type GTFs in sp3-carbon, as well as hydrogen. The optimized molecular exponents give a flexible description of σ and π bonds in C-C chemical bonds induced by the difference of hybrid states. We also confirmed the efficiency of our calculation concerning the total energy and geometrical parameters in comparison with the results using the high quality basis sets.
The geometric isotope effect (GIE) on low barrier hydrogen-bonded systems of acetic acid dimer, formic acid dimer, and their anion clusters is analyzed by HF and hybrid DFT levels of the multi-component molecular orbital (MC_MO) method, which directly includes nuclear quantum effect of proton/deuteron. Our optimized geometries for both neutral and anionic species with HF level of MC_MO method have reproduced the overall tendency of the GIE of the corresponding experimental ones. On the other hand, the results for anionic clusters with hybrid BHandHLYP functional of MC_MO method give poor agreement due to the underestimation of the barrier height of hydrogen-bonding. Our multi-component analysis clearly demonstrates that the hydrogen-bonding interaction energy is strongly affected by the distribution of nuclear wavefunction.
Previously, our research group carried out molecular orbital studies of the interaction between MgATP and the myosin motor domain using the PM3 method of MOPAC97. In this study, we present recalculation results obtained using the PM6 method of MOPAC2009 and discuss the problems that may be encountered in further studies. Previous research carried out by our group in this field is introduced briefly. Furthermore, it is proposed that the increase in the distance between Pγ and the bridging O atom bound to Pβ initiates the dissociation of γ-phosphate from ATP. Our results demonstrate that semi-empirical molecular orbital methods are useful in studying the chemical reactions underlying the initial step of ATP hydrolysis in the myosin motor domain.
The development of functional liposomes using phospholipid liposomes has been performed for some time; however, from the perspectives of efficiency and safety, it is important that the fusion or aggregation of liposomes be controlled and regulated. Phospholipids are ubiquitous in biological membranes; among them, phosphatidylcholines (PC) are commonly used in studies on bilayer membranes and functional liposomes. In addition, the aggregation/disaggregation phenomena of PC liposomes are thought to be caused by heat fluctuations in the membrane during gel-liquid crystal transition. Moreover, a dipole is present in the hydrophilic moiety of PC molecules. Therefore, it is considered that the aggregation phenomenon of phospholipid liposomes might be due to heat fluctuations in the membranes and also the effect of local electric forces caused by the dipole. Therefore, in order to examine the three-dimensional conformation of phospholipid molecules, we calculated the total energy of PC molecules using the molecular orbital method, and 3 optimized structures were found. These structures correlated well with those obtained from other experiments. This shows that the analysis of the molecular structure of phospholipids using the molecular orbital method can be an effective tool to elucidate the interactions between membranes and the properties and functions of phospholipid liposomes.