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Reference Guide
This manual provides a detailed list of all the input keywords and options available in Radioss.
Engine Input
This manual provides a list of all the solution definition keywords and options available in Radioss.
Implicit Solution
Engine Keyword In this group keywords are used for implicit solution. Different implicit solver methods (linear, nonlinear, buckling or quasi-static), different options to control the implicit computation, different implicit time step method and implicit output messages are introduced.
Implicit Controls
Engine Keyword
/IMPL/GSTIF/OFF
Engine Keyword Geometrical stiffness matrix will not be used for implicit nonlinear calculation.

Welcome
What's New
View new features for Radioss 2025.1.
Overview
Radioss^® is a leading explicit finite element solver for crash and impact simulation.
Tutorials
Discover Radioss functionality with interactive tutorials.
User Guide
This manual provides details on the features, functionality, and simulation methods available in Altair Radioss.
Reference Guide
This manual provides a detailed list of all the input keywords and options available in Radioss.
- File Extensions and Formats
  Current Radioss format uses the 12x extension format.
- Single File Input
  This format allows running either Starter or Engine with the same file.
- New Keywords in 2025
  New and modified features in Radioss.
- Starter Input
  This manual provides a list of all the model definition keywords and options available in Radioss.
- Engine Input
  This manual provides a list of all the solution definition keywords and options available in Radioss.
  - Engine Keywords Syntax
  - General Controls
    Engine Keyword In this group keywords are used to generally define the output style, control of computation, damping and ALE setup during the computation.
  - Deletion/Input Modifications
    Engine Keyword In this group, keywords are used for modification (deletion or add options) in Engine.
  - Animation and Post-processing Output Files
    Generates animation files containing results according to the keywords specified. The keywords are specified in the topics within Animation and Post-processing Output Files.
  - Implicit Solution
    Engine Keyword In this group keywords are used for implicit solution. Different implicit solver methods (linear, nonlinear, buckling or quasi-static), different options to control the implicit computation, different implicit time step method and implicit output messages are introduced.
    - Implicit Controls
      Engine Keyword
      - /IMPL/AUTOSPC
        Engine Keyword A zero stiffness DOF will be constrained automatically.
      - /IMPL/DIVERG/n
        Engine Keyword Divergence criterion with successive increasing residual number for nonlinear analysis.
      - /IMPL/DYNA/1
        Engine Keyword Describes the implicit dynamics with $α$ -HHT method.
      - /IMPL/DYNA/2
        Engine Keyword Describes the implicit dynamics with a general Newmark method.
      - /IMPL/DYNA/DAMP
        Engine Keyword Rayleigh structure damping with dynamic implicit analysis.
      - /IMPL/GSTIF/OFF
        Engine Keyword Geometrical stiffness matrix will not be used for implicit nonlinear calculation.
      - /IMPL/INTER/KCOMP
        Engine Keyword Describes the stiffness matrix; due to contact interfaces will be assembled completely.
      - /IMPL/INTER/KNONL
        Engine Keyword Defines including some contact nonlinearities in PCG linear solver.
      - /IMPL/LINEAR/INTER
        Engine Keyword Implicit linear analysis will take into account contact interfaces.
      - /IMPL/LSEARCH/OFF
        Engine Keyword Deactivating line search for nonlinear analysis.
      - /IMPL/LSEARCH/n
        Engine Keyword Line search methods for nonlinear analysis.
      - /IMPL/MONVOL/OFF
        Engine Keyword Describes the stiffness of gas in monitored volume type 3 (tire modeling).
      - /IMPL/MUMPS/AUTOCORE
        Engine Keyword MUMPS solver will automatically switch from in-core memory to out-of-core memory.
      - /IMPL/MUMPS/OUTCORE
        Engine Keyword MUMPS solver will use minimum in-core memory.
      - /IMPL/NCYCLE/STOP
        Engine Keyword The computation will be stopped, if Ncy_max is reached.
      - /IMPL/PREPAT
        Engine Keyword Describes the implicit option for pre-conditioning.
      - /IMPL/PSTIF/OFF
        Engine Keyword Deactivating load (pressure) stiffness matrix.
      - /IMPL/RREF/INTERF/n
        Engine Keyword Different reference force residual computations used for stop criteria on nonlinear analysis with contact.
      - /IMPL/RREF/LIMIT
        Engine Keyword Define reference force residual limit values used in stop criteria for implicit nonlinear.
      - /IMPL/RREF/OFF
        Engine Keyword Deactivation reference residual option and using the previous one in stop criteria for implicit nonlinear analysis.
      - /IMPL/SHPOFF
        Engine Keyword The full projection of QEPH shell element for warped elements will be deactivated to keep consistency between the stiffness matrix and the internal force computations.
      - /IMPL/SHPON
        Engine Keyword The full projection of QEPH shell element for warped elements will be activated for stiffness matrix to keep consistency with the internal force computations.
      - /IMPL/SINIT
        Engine Keyword Describes the initial stresses that will be imposed gradually.
      - /IMPL/SPRBACK
        Engine Keyword Implicit spring-back will be run in this session.
      - /IMPL/SPRING
        Engine Keyword Describes the linear or nonlinear stiffness choices for nonlinear spring in implicit nonlinear analysis.
    - Implicit Solvers
      Engine Keyword
    - Implicit Time Step
      Engine Keyword
    - Implicit Model Check
      Engine Keyword
    - Implicit Output Messages
      Engine Keyword
  - Kinematics and Loading
    Engine Keyword In this group, keywords are used to define the kinematic condition (constrain/release rotation or translation DOF) and initial condition (rotational/translational velocity).
  - Time Step
    Engine Keyword In this group, keywords are used to define different time step control (STOP, KILL, CST or DEL, and so on) for different entities (shell, brick, beam, airbag, interface, and so on) during the computation.
  - Initial State
    Engine Keyword In this group, keywords are used to output different info (like stress, strain, orthotropy direction, temperature, and so on) in a state file (STA-File).
- LS-DYNA Input
  This manual provides a list of LS-DYNA input files available in Radioss.
- Multi-Domain
  This manual contains the description of the keywords for the Radioss Multi-Domain.
- Obsolete Keywords
  Keywords that are still supported, but no longer maintained (*) are considered obsolete.
- Other Files
- Definition
Example Guide
This manual presents examples solved using Radioss with regard to common problem types.
Verification Problems
This manual presents solved verification models.
Frequently Asked Questions
This section provides quick responses to typical and frequently asked questions regarding Radioss.
Theory Manual
This manual provides detailed information about the theory used in the Altair Radioss Solver.
User Subroutines
This manual describes the interface between Altair Radioss and user subroutines.

/IMPL/GSTIF/OFF

Engine Keyword Geometrical stiffness matrix will not be used for implicit nonlinear calculation.

Format

/IMPL/GSTIF/OFF