Tech Articles

ACMS vs Lanczos ( SOL 103 )

Tech Articles ID    KB8022076
Status:    Published
Published date:    04/23/2015
Updated:    06/29/2015
Reported In:   MSC Nastran - MSC Nastran - Standard (SOL 1 through 200)

Abstract (Question)


  • Lanczos is generally faster than ACMS for smaller models and models with fewer modes.
  • ACMS is generally faster than Lanczos models with lots of modes.
  • ACMS will find a few less modes than Lanczos.

Description (Answer)

Below are graphs for number of modes vs elapsed time for various models with ACMS and Lanczos:


  • All runs above were made with MEM=MAX MODE=I8 on a system with 64Gb of RAM.
  • All runs above were made F1 and F2 on the EIGRL entry, rather than ND

Lanczos is the default eigenvalue solver. To use ACMS add the following line in the executive section:


The EIGRL entry is used to select the number of Frequencies:

     EIGRL  SID     F1      F2      ND      MSGLVL  MAXSET  SHPSCL  NORM
     EIGRL  300     -0.1    300.0                           1.0


  • ACMS replaces the full eigensolution on all A-set DOF with a modal synthesis of automatically generated components. This method means we have an approximation to the full Lanczos eigensolution. This technique will show fewer eigenvalues. The lower modes will have very good fidelity. If this is a big concern, you can increase the maximum frequency used internally by ACMS. The default is two times your EIGRL F-max. You can increase this by modifying your DOMAINSOLVER entry:
    Where 'value' is any real number, e.g. 2.5 or 3.0.  The default value is 2.0, which has been found to produces good results.
    • ACMS has a limitation of DMP=8
    • The EIGRL entry needs F2 for ACMS.


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