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Next: Fitting to ab initio Up: Interatomic potentials Previous: Long-range forces

But do we really need potentials?

  About ten years ago, Car and Parrinello [34] developed a powerful method allowing to perform MD simulations where the ions are still moved classically, but under the action of forces obtained by solving the electronic structure problem, thus eliminating the need for empirical potentials at the expense of much larger computational requirements. This important development spawned one of the most important research lines in today's condensed matter physics, known as first-principles (or ab initio) molecular dynamics (FPMD), or simply as Car-Parrinello method.

Will the existence of the certainly more accurate FPMD technique imply the death of classical molecular dynamics based on empirical potentials? It is safe to say that the answer is no. While at present classical MD is being run on systems containing million of particles on the most powerful computers, or for simulation times of the order of nanoseconds (10-9 s), the current limit for FPMD is of the order of a thousand atoms, and its typical simulation times are measured in picoseconds (10-12 s). This means that several problems requiring large sizes and/or large times--such as many of those mentioned in §1.4--can be attacked only by classical methods. Even if the speed of computers keeps increasing at a breathtaking pace, so does the size of problems of interest, and it is very likely that all the MD techniques that we know today will remain in use for quite some time from now, and new ones may appear.

In fact, classical MD and Car-Parrinello MD already represent the extremes of a spectrum of MD methods, differing in the degree of approximation used to solve the electronic problem in order to obtain atomic forces. In particular, a promising area is that of tight-binding molecular dynamics (TBMD), where systems containing thousand of particles are now being simulated thanks to the recent development of O(N)-scaling computer codes.

The reader that wants to be introduced to the fast-growing areas of simulations from first-principles could start from the review articles present in the literature [35,36,37].


next up previous contents
Next: Fitting to ab initio Up: Interatomic potentials Previous: Long-range forces
Furio Ercolessi
9/10/1997