# Environment Variables¶

With a bash shell, these are set by, e.g. (on ubuntu)

extern NEURONHOME=/where/neuron/was/installed/share/nrn


## NRNUNIT_USE_LEGACY¶

When set to 1, legacy unit values for FARADAY, R, and a few other constants are used. See nrn/share/lib/nrnunits.lib.in lines which begin with @LegacyY@, nrn/src/oc/hoc_init.c in the code section static struct { /* Modern, Legacy units constants */, and nrn/src/nrnoc/eion.c.

When set to 0, (default), values from codata2018 are used. See nrn/share/lib/nrnunits.lib.in lines that begin with @LegacyN@ and nrn/src/oc/nrnunits_modern.h.

Switching between legacy and modern units can also be done after launch with the top level HOC function nrnunit_use_legacy().

The purpose of allowing legacy unit values is to easily validate results of old models (double precision identity).

This environment variable takes precedence over the CMake option NRN_DYNAMIC_UNITS_USE_LEGACY.