IBRION = -1 | 0 | 1 | 2 | 3 | 5 | 6 | 7 | 8
| Default | |
| for NSW=0 or NSW=1 |  |
| else | 0 |
IBRION determines how the ions are updated and moved. For IBRION=0, a molecular dynamics is performed, whereas all other algorithms are destined for relaxations into a local energy minimum. For difficult relaxation problems it is recommended to use the conjugate gradient algorithm ( IBRION=2), which presently possesses the most reliable backup routines. Damped molecular dynamics ( IBRION=3) are often usefull, when starting from very bad initial guesses. Close to the local minimum the RMM-DIIS ( IBRION=1) is usually the best choice. IBRION=5 and IBRION=6 are using finite differences to determine the second derivatives (Hessian matrix and phonon frequencies), whereas IBRION=7 and IBRION=8 use density functional perturbation theory to calculate the derivatives.