> Hi, siesta users: > I noticed that one line in Siesta manual, saying: > "The Ordern(N) subsystem is quite fragile and only > works for systems with clearly separated occupied > and empty states." > > Is there any obvious reason that it cannot handle > metallic systems? > > Best regards, > Bing >
Sure, Bing, it is. In metal, the states which are close to the Fermi energy may shift back and forth between occupied and unoccupied domains. On each iteration, the eigenvalues are ordered by their increased values, and the AUFBAU PRINCIPLE applied - the so many lowest states (counted over the Brillouin zone) are declared OCCUPIED (and contribute to the density in the next iteration), and the rest thrown out. The Order(N) approach (at list as it is implemented in Siesta, or more general, as far as I remember) avoids diagonalization; instead it works directly on orbitals or density matrix elements, iterating them according to a certain criterion. Hence the method does not know eigenvalues and goes ahead for density, total energy, and other related properties. (But does NOT provide the density of states, say). Therefore the method cannot systematically order the eigenvalues and find out where the occupied ones end. Instead, it needs to know this in advance. In practice, in addition to the number of electrons (which is known anyway), the chemical potential has to be provided IN ADVANCE as a measure to identify the occupied states. If you have a gap and safely put the chemical potential inside, the calculation may go stable. If you have a metal, it ends up in a mess; the iteration algorithms go astray. Best regards Andrei Postnikov
