Biomedical subjects

Lothar Schäfer

Publications and source records attributed to Lothar Schäfer.

3 recordsLinked to original sources

PubMed2005

Regularization of moving boundaries in a laplacian field by a mixed Dirichlet-Neumann boundary condition: exact results.

The dynamics of ionization fronts that generate a conducting body are in the simplest approximation equivalent to viscous fingering without regularization. Going beyond this approximation, we suggest that ionization fronts can be modeled by a mixed Dirichlet-Neumann boundary condition. We derive exact uniformly propagating solutions of this problem in 2D and construct a single partial differential equation governing small perturbations of these solutions. For some parameter value, this equation can be solved analytically, which shows rigorously that the uniformly propagating solution is linearly convectively stable and that the asymptotic relaxation is universal and exponential in time.

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PubMed2004

Solids modeled by ab initio crystal field methods, part 22: the Fock matrix transformed supermolecule model and the structure determination of D-erythronic acid-3,4-carbonate.

To eliminate the cutoff effects in the supermolecule model (SM), the Fock matrix transformed supermolecule model (FTSM) is developed. In this model a cyclization of the cluster is performed by means of transformations of elements of the Fock matrix, thereby restoring the translational symmetry. Besides this fundamental enhancement, significant CPU time savings are realized because, in this new procedure, not all Fock matrix elements need to be calculated. The method is applied in a study of the structure of D-erythronic acid-3,4-carbonate in the crystal phase, where the new model is compared to the supermolecule model as well as the XRD experiment. The results are found to be in good agreement with experimental data.

Algorithms↗

PubMed2002

Coherent scattering function in the reptation model: analysis beyond asymptotic limits.

We calculate the coherent dynamical scattering function S(c)(q,t;N) of a flexible chain of length N, diffusing through an ordered background of topological obstacles. As an instructive generalization, we also calculate the scattering function S(c)(q,t;M,N) for the central piece of length M infinity, M fixed. The results do not support the model of a "Rouse chain in a tube." We trace this back to the nonequilibrium initial conditions of the latter model. Second, in the limit of long chains (M=N--> infinity ) and times large compared to the internal relaxation time (t/N(2)--> infinity ), our theory reproduces the results of the primitive chain model. This limiting form applies only to extremely long chains, and for chain lengths accessible in practice, effects of, e.g., tube length fluctuations are not negligible.

Journal Article↗