Nc != 3 in Chroma

These notes are checked into the docs subdirectory of Chroma.

Chroma for Nc != 3

The chroma library is built on top of qdp++. The number
of colours in qdp++ is a global parameter that can be
set at compile time.

There are various part of chroma where there are functions that
are hard wired for SU(3). For example: baryons because of those pesky epsilon factors
  • baryons because of those pesky epsilon factors
  • any code that moves from Lie algebra to the group. For example stouting
Chroma has been used to compute quark propagators
and generate gauge configurations for Nc > 3 (arXiv:0806.2278).
At the moment the hmc doesn't work for arbitary


To compile chroma for the SU(6) gauge group for example, you first need to compile qdp++ for correct number of colours

./qdp++/configure --enable-parallel-arch=scalar  --enable-Nc=6 \
                  [your normal compilation options]

The default value for Nc is 3.

Here I assume that you want to just compile the pure chroma
library. Other libraries such as BAGEL may not work with chroma
for an arbitary number of colours.

Developer guide

The essential technique to get the code to compile
for SU(N_c) is to not to include any code hardwired for
SU3 in the compile.

if ( Nc != 3 ){    /* Code is specific to Ns=4 and Nc=3. */
    QDPIO::cerr<<" code only works for Nc=3 and Ns=4\n";
    QDP_abort(111) ;
#if QDP_NC == 3
// the rest of the code for SU(3)

      LatticePropagator q1_tmp;

      q1_tmp = zero ;
      return q1_tmp ;

See the functions below for example:


Sometimes the preprocessor can be used to get rid of the
entire file. Other times the preprocessor directives have to
go into the body of the function.

Some of us really hate preprocessor directives, because
they are like poisonous weeds that can strangle a code.
However, in this case there are not too many directives used.

I don't believe there has been any thought into designing the
code, so that it can do baryons for large N_c. Some effective field
theory people are interested in large N_c mass splittngs for baryons


Hey dude, remember that Nc=3, N=0, and the coupling runs
in the real world. Please enjoy your theoretical speculation