Time reversible and symplectic integrators for molecular dynamics simulations of rigid molecules.

Molecular dynamics integrators are presented for translational and rotational motion of rigid molecules in microcanonical, canonical, and isothermal-isobaric ensembles. The integrators are all time reversible and are also, in some approaches, symplectic for the microcanonical ensembles. They are developed utilizing the quaternion representation on the basis of the Trotter factorization scheme using a Hamiltonian formalism. The structure is similar to that of the velocity Verlet algorithm. Comparison is made with standard integrators in terms of stability and it is found that a larger time step is stable with the new integrators. The canonical and isothermal-isobaric molecular dynamics simulations are defined by using a chain thermostat approach according to generalized Nosé-Hoover and Andersen methods.

Molecular dynamics study of mesophase formation using a transverse quadrupolar Gay-Berne model.

The effect of the transverse electric quadrupole moment on the formation of liquid-crystal mesophases was investigated by means of a computer-simulation study. A simple model of the molecular interaction employed the addition of a transverse, point quadrupole to a Gay-Berne potential. The transverse quadrupole was seen to raise the temperature of onset of the smectic phase. A large magnitude quadrupole stabilized the smectic-A phase over the temperature range studied compared to a Gay-Berne reference fluid. The presence of a large quadrupole stabilized cubic smectic phases rather than the more usual hexagonal smectic-B phases.

Mammographic follow-up of nonpalpable low-suspicion breast abnormalities: one versus two views.

A retrospective study involving 498 women with a total of 666 breast lesions was undertaken to determine the relative efficacy of one- and two-view mammography in the follow-up evaluation of "low-suspicion" abnormal mammographic findings. These abnormalities consisted of well-defined masses (47.1%), well-defined punctate microcalcifications (20.9%), and parenchymal asymmetry (32.0%). Confidence in the adequacy of the single-view follow-up was high in 91% of cases. The addition of the second mammographic view changed the one-view interpretation in approximately 1% of all cases. Two cancers were detected during the initial follow-up period. Both cancers were detected with single-view and standard two-view follow-up examinations, with high confidence. In this controlled retrospective study, the single-view follow-up examination was adequate for follow-up of most low-suspicion mammographic abnormalities. Monitoring by physicians, however, would be necessary to prevent an unacceptable number of patient recalls, which could make the one-view follow-up study impractical to use in some practices.