For any continuous structure there are theoretically infinite natural frequencies, but generally engineers dealing with real structures only need to consider those natural frequencies that are likely to occur in the real world. These are usually the lowest natural frequencies for the structure. Multiframe4D calculates from the lowest natural frequency upwards to a possible of 20 natural frequencies.
 
To carry out a modal analysis using computational methods we approximate the continuous system by discretising the structure into a finite number of degrees of freedom. Each degree of freedom allows us to calculate one natural frequency. The more degrees of freedom the more accurate the solution.
 
A good rule of thumb is that the minimum number of degrees of freedom should be at least double the required natural frequencies. Multiframe enforces this rule.
 
Each node has 6 degrees of freedom. A structure with 2 nodes, one fixed, would eliminate the degrees of freedom for that node, so you are left with 6 degrees of freedom in total. Thus considering the rule above, Multiframe4D will only return a maximum of 3 natural frequencies.
 
The solution is to use the subdivide command to increase the number of nodes in the structure and therefore the degrees of freedom. If you experiment with different levels of discretisation you will notice improvements in the accuracy of the solution for the higher natural frequencies as the number of nodes is increased.