[CPMD-list] Fd and LR in vibrational analysis

[Juerg Hutter]

Hi

Just to add to my previous mail and the mail by Bala

VIBRATIONAL ANALYSIS FD

uses finite difference (2 points) of first derivatives.
This way you need 6N SCF calculations to calculate the
dynamical matrix. Calculating second derivatives by
finite differences would require 9N^2 SCF calculations.

VIBRATIONAL ANALYSIS LR

calculates the dynamical matrix analytically (almost).
To do this you need the linear response orbitals
that are solutions to the coupled KS equation.
There are 3N perturbations, meaning you have to
solve 3N CPKS systems iteratively. One iteration
of the CPKS iteration is about the same time as
an SCF iteration in CPMD. Therefore the two schemes
(LR and SD) need about the same total amount of CPU
time. The actual relative timings is depending on
the convergence of the two schemes and might differ
from system to system.

regards

Juerg Hutter

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Juerg Hutter                   Phone : ++41 44 635 4491
Physical Chemistry Institute   FAX   : ++41 44 635 6838
University of Zurich           E-mail: hutter at pci.unizh.ch
Winterthurerstrasse 190
CH-8057 Zurich, Switzerland
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On Tue, 28 Nov 2006, Yong Zhang wrote: