# What FCIQMC codes are out there?

Full configuration interaction quantum Monte Carlo seems like it is poised to overtake DFT in some applications pretty soon. I am curious if there is any freely available implementation of the method, as I cannot seem to find one.

• You might want to look at the computational cost of FCIQMC relative to DFT before making such a statement. It is also prudent to consider that there are maybe 10 papers published demonstrating FCIQMC and they are all by people who are funded to develop these methods. On the other hand, there are thousands of papers demonstrating the use of DFT and clearly not all of them were written by Walter Kohn and the like. – Jeff Hammond Sep 30 '13 at 3:27

These methods are intended to compete with wavefunction methods on accuracy but with a lower computational complexity. For example, i-FCIQMC has been reported for ethene and butadiene molecules (http://doi:10.1021/ct300486d), that otherwise are impossible to treat with Full Configuration Interaction. In fact the later cannot go beyond dinitrogen ($N_2$) with a decent basis set. On the other hand this is not always possible. For strongly correlated systems you will have to explore almost all the Hilbert space, then you gain only a slight or no benefit from using them. Improvements are part of current research.