The total number of segments.
The choice for the total (that is, final) number of segments (that is, subdivisions) is:
(1) 0 -the usual choice when setting up a new simulation. In this case the program simply finds the sum of all the subdivision numbers already entered for the separate electrodes, which we can call the 'initial number' and which will be stated on the screen print-out..
(2) A number less than the 'initial number' (for CPO2D only), which can be useful if you want a smaller total number but do not want to edit the individual subdivision numbers. For CPO3D use the supplied external program reduce.
(3) A number equal to the 'initial number', in which case the program will not change the segments.
(4) A number greater than the 'initial number'. The action taken by the program then depends on another requested number, the number of steps. If the number of steps is 1 then the program simply subdivides all the segments by approximately the same amount to try to reach the requested final number. If the number of steps is greater than 1 then iterative subdivision (also called adaptive segmentation) takes place (and then a 3rd number is used). This option should be used with caution. It is usually better to retain control over the process of subdivision. The final number given by the program might be slightly smaller or larger than the number requested here.
Important advice on increasing the number of segments for 3D systems (that is, the fourth choice):
Ideally the increase in the number of segments should be by a factor of 2 or 4 or 8 etc (that is, 2**n, 2 to the power of an integer). The reason for this is that the electrodes are firstly subdivided into basic triangular or rectangular segments and then these segments are further subdivided to try to achieve the requested final total number. By using a factor 2**n all the triangular or rectangular segments can be subdivided by the same amount (see footnote below). If the increase is not by a factor that has the form 2**n then the segments that are earliest in the list of segments will be subdivided more than the later segments.
Further note: The 4th choice is not allowed for 3D systems (that is, the segments cannot be changed) if the 'inscribing correction' has been applied to any of the segments, as illustrated in figure 2.6 in the Users Guide.
Footnote: A basic triangular segment is subdivided by firstly bisecting its longest side to create 2 triangles, then similarly splitting those 2 to create 4, etc. A basic rectangular segment is subdivided by firstly bisecting its longest sides to create 2 rectangles, then similarly splitting those 2 to create 4, etc.
Return to general note on the total number of segments, 'iterative subdivision', choice of charge inaccuracy.