Dear PWSCF users and developers,
I am wondering how can one find machine accuracy? I am having
some symmetry problem, and for that I want to solve eigen value
problem with the best accuracy. In the lapack library it says-
ABSTOL (input) DOUBLE PRECISION
* The absolute error tolerance for the eigenvalues.
* An approximate eigenvalue is accepted as converged
* when it is determined to lie in an interval [a,b]
* of width less than or equal to
*
* ABSTOL + EPS * max( |a|,|b| ) ,
*
* where EPS is the machine precision. If ABSTOL is less than
* or equal to zero, then EPS*|T| will be used in its place,
* where |T| is the 1-norm of the tridiagonal matrix obtained
* by reducing A to tridiagonal form.
*
* Eigenvalues will be computed most accurately when ABSTOL is
* set to twice the underflow threshold 2*DLAMCH('S'), not zero.
* If this routine returns with INFO>0, indicating that some
* eigenvectors did not converge, try setting ABSTOL to
* 2*DLAMCH('S').
the last paragraph is useful. so i looked up DLAMCH-
DOUBLE PRECISION FUNCTION DLAMCH( CMACH )
The input is CMACH, I can specify various input variables for 'CMACH' to
get output, but what variable is returned as output, as there is only one
in this subroutine? Would you mind having a look at this- its in
espresso4.0.4/flib/dlamch.f
espresso4.0.4 can be replaced by whatever version you are using.
Regards,
Manoj Srivastava
University of Florida
Gainesville, USA
