There are a large number of molecular dynamics packages currently available and, while performance varies across each piece of software and the type of method and approximations employed, they all effectively simulate on the nanosecond timescales out to limits of tens to hundreds of nanoseconds. In the next few years this will likely be extended into the microsecond regime, but we are unlikely to see any form of routine simulation above several microseconds of simulation time for many years to come. It should be noted that simply extending the length of time for which a system is simulated will only go so far toward improving the results. For example, to simulate rare events, it will always be necessary to carry out more complex sampling using some of the methods described above. Thus, it will be essential in the coming years that effort is focused not just on improving the performance of MD codes for simple linear MD simulations, but also on all the advanced sampling approaches that have been developed.

To give some context to the types of resource requirements simulations of cellulose require, a simulation of a cellulose bundle in explicit water comprising 408 000 atoms runs at a rate of 2.7 ns/day on 768 processors of San Diego Supercomputer Center’s DataStar (IBM Power 4) supercomputer using version 9 of Amber’s PMEMD molecular dynamics software. Thus, a 20 ns long simulation requires access to such a supercomputer for 7.4 days and utilizes a total of approximately 136 000 CPU hours.