www.neuron.yale.edu

I am not an experimentalist but sometimes I try to simulate real experimental results. The SEClamp in NEURON simulates a whole-cell voltage clamp mode, right? So what would I need to do to simulate cell-attached mode? I know that in whole cell mode you have access to the whole cell, while in cell attached mode, the membrane remains intact and you are reading the current through the membrane.

Any help would be appreciated.

AFAIK "whole cell" and "cell attached" mean the same thing. Perhaps you are
concerned with the difference between "whole cell" and "perforated patch"
recording? In practical terms, the difference is that series resistance
between the recording apparatus and the cell would be expected to be
larger with perforated patch than with "whole cell" recording (but series R
can get pretty big with whole cell recording, if the electrode gets plugged).
Generally, series resistance < 10 megohms is regarded as desirable;
useful results can be obtained with higher values.

To simulate the effect of series resistance under voltage clamp, just use
an SEClamp and increase its rs. To simulate the effect under current clamp,
it is most convenient to use the Linear Circuit Builder to construct a circuit
that consists of the cell and a current source with a resistor that connects
the two. Fancier circuits can be constructed, e.g. with an equivalent T
representation of the electrode (the top of the T is the series R split into
two equal halves, and the vertical bar is the electrode capacitance to
ground). One can even add "capacitance compensation" and "series
resistance compensation" circuits to explore the benefits and limits of
trying to compensate for electrode properties.