Two direct solvers and one iterative solver are available for the solution of the set of equations.
In finite element analysis, a problem is represented by a set of algebraic equations that must be solved simultaneously. There are two classes of solution methods: direct and iterative.
Direct methods solve the equations using exact numerical techniques. Iterative methods solve the equations using approximate techniques where in each iteration, a solution is assumed and the associated errors are evaluated. The iterations continue until the errors become acceptable.
The software offers the following choices:
Automatic 
The software selects the solver based on the study type, analysis options, contact conditions, etc. Some options and conditions apply only to either Direct Sparse or FFEPlus. 
Direct Sparse 

FFEPlus (iterative) 

Large Problem Direct Sparse 
Available for static and nonlinear studies. The solver can handle cases where the solution is going out of core. 
Choosing a Solver
The Automatic choice for a solver is the default option for Static, Frequency, Buckling, and Thermal studies.
In the case of multiarea contact problems, where the area of contact is found through several contact iterations, the Direct Sparse solver is preferred.
While all solvers are efficient for small problems (25,000 DOFs or less), there can be big differences in performance (speed and memory usage) in solving large problems.
If a solver requires more memory than available on the computer, then the solver uses disk space to store and retrieve temporary data. When this situation occurs, you get a message saying that the solution is going out of core and the solution progress slows down. If the amount of data to be written to the disk is very large, the solution progress can be extremely slow. In these cases (for static and nonlinear studies), use the Large Problem Direct Sparse.
The following factors help you choose the proper solver:
Size of the problem 
In general, FFEPlus is faster in solving problems with degrees of freedom (DOF) over 100,000. It becomes more efficient as the problem gets larger. 
Computer resources: Available RAM and number of CPUs (core or processors) 
The Direct Sparse solver requires about 10 times more RAM than the FFEPlus solver. It becomes faster with more memory available on your computer. The Large Problem Direct Sparse leverages multicore processing capability and improves solution speed for static and nonlinear studies. 
Material properties 
When the moduli of elasticity of the materials used in a model are very different (like Steel and Nylon), then iterative methods could be less accurate than direct methods. The direct solvers are recommended in such cases. 
Analysis features 
Analysis with No Penetration contacts and Bonded contacts enforced using constraint equations will typically solve faster with the direct solvers. 
Depending on the study type, the following recommendations apply:
Static 
Use the Direct Sparse and Large Problem Direct Sparse when you have enough RAM and multiple CPUS for solving:
 Models with No Penetration contact, especially when you turn on the friction effects.
 Models with parts that have widely different material properties.
 Mixedmesh models
For a linear static analysis, the Direct Sparse solver requires 1 Gb of RAM for every 200,000 degrees of freedom (dof). The iterative FFEPlus solver is less demanding on memory (approximalety 2,000,000 dof / 1 Gb of RAM).

Frequency and Buckling 
Use the FFEPlus solver to calculate any rigid body modes. A body without any restraints has six rigid body modes.
Use the Direst Sparce solver for:
 Considering the effect of loading on the natural frequencies
 Models with parts that have widely different material properties.
 Models where incompatible mesh is bonded using constraint equations.
 Adding soft springs to stabilize inadequately supported models (buckling studies).
Simulation uses the Subspace iteration method as the eigenvalue extraction method for the Direst Sparse solver, and the Lanczos method for the FFEPlus solver. It is more efficient to use Lanczos with iterative solvers like FFEPlus. Subspace can utilize the back and forth substitution of the Direct (Sparse) solvers within its iteration loop to evaluate the eigenvectors (only needs to decompose the matrix once). That is not possible with iterative solvers.

Thermal 
Thermal problems have one degree of freedom (DOF) per node, and hence their solution is usually much faster than structural problems of the same number of nodes. For very large problems (larger than 500.00 dofs), use the Large Problem Direct Sparse, or the FFEPlus solver. 
Nonlinear 
For Nonlinear studies of models that have more than 50,000 degrees of freedom, the FFEPlus solver is more effective in giving a solution in a smaller amount of time. The Large Problem Direct Sparse solver can handle cases where the solution is going out of core. 
Solver Status
The Solver Status window appears when you run a study. In addition to progress information, it displays:
 Memory usage
 Elapsed time
 Studyspecific information such as degrees of freedom, number of nodes, number of elements
 Solver information such as solver type
 Warnings
All studies that use the FFEPlus (iterative) solver let you access the convergence plot and solver parameters. The convergence plot helps you visualize how the solution is converging. The solver parameters let you manipulate the solver iterations so that you can either improve accuracy or improve speed with less accurate results. You can use the solver's preset values or change:
 Maximum number of iterations (P1)
 Stopping threshold (P2)
To improve accuracy, decrease the stopping threshold value. In slowly converging situations, you can improve speed with less accurate results by increasing the stopping threshold value or decreasing the maximum number of iterations.