Research Highlights:
O(N) Krylovsubspace method for largescale systems
Figure: Underlying idea of the O(N) Krylov subspace method.
The computational cost of conventional density functional theories (DFT) calculations scales as the third power
of the number of atoms, which means that the computational time becomes a thousand times longer than
that for an original system when you try to calculate a ten times larger system. This presents an apparent
difficulty to researchers who deal with realistic largescale problems. On another front, it is known that
chemical properties of functional groups in molecules are mostly determined by local atomic arrangement.
The chemical intuition may enable us to develop an efficient method which determines the local electronic
structure by making use of mainly shortrange information. Along this line, we have developed an efficient
and robust O(N) method by unifying two ideas of the divideconquer and Krylov subspace methods, shown in
the upper figure, whose computational cost scales only linearly as a function of the number of atoms [1].
Figure: Difference charge density for a (10,0) finite sized zigzag
carbon nanotube on aluminum surface calculated by the O(N) method.
The O(N) computational cost is guaranteed by the idea of the divideconquer method, and the reduction of
dimension by the Krylov subspace method provides a small computational prefactor. The practical algorithm
is cast to solving a series of embedded cluster problems defined in Krylov subspaces.
The O(N) method is expected to be a promising approach for realization of largescale ab initio
calculations for a wide variety of materials, since it is applicable to not only insulators
but also metals in a single framework. The bottom figure shows difference charge density caused by
the interaction between Al (001) and a (10,0) zigzag carbon nanotube, calculated by the O(N) method.
We have been applying the O(N) method to various interface problems such as electrodeelectrolyte
and metalinsulator.

"O(N) Krylovsubspace method for largescale ab initio electronic structure calculations",
T. Ozaki, Phys. Rev. B 74, 245101 (2006).