Hello,
the Eigen-6S model is the default model if you use the default "orekit-data.zip".
If you want to use a specific model, I think it's better to load it explicitly.
I'm a basic user of Orekit so my method is probably not the best one... But I give you the extract of the piece of code I use to load a specific model.
/**
* Load data from ICGEM file.
*/
public void loadDataBase(String filename) throws OrekitException, IOException, ParseException {
// Prepare the provider
if (filename.isEmpty()) filename = "egm96.gfc";
GravityFieldFactory.clearPotentialCoefficientsReaders();
GravityFieldFactory.addPotentialCoefficientsReader(new ICGEMFormatReader(filename, true));
provider = GravityFieldFactory.getConstantNormalizedProvider(this.degree, this.order);
}
As far as I can remember, the handling of the data sets is explained in the Wiki, in the "Configuration" section.
I hope it will help you to understand how to load your geopotential model.