Do you know what the column names are that are returned by periodogram_matrix() function I thought they were omega and spectral density, but the spectral density from periodogram() function seems to output something different than periodogram_matrix() function. Below is some output from the periodogram() and periodogram_matrix(), which may not be required to answer the question Thank you riley this line-> periodogram(0, -1, dset, OPT_O, prn); shows graph and displays the following omega scaled frequency periods spectral density 0.00617 1 1018.00 1.0926e+05 0.01234 2 509.00 1.0554e+05 0.01852 3 339.33 99587 0.02469 4 254.50 91770 0.03086 5 203.60 82549 ............ ... ............ .............. ............ ... ............ .............. 3.11690 505 2.02 709.98 3.12308 506 2.01 698.51 3.12925 507 2.01 689.31 3.13542 508 2.00 683.36 3.14159 509 2.00 681.30 the lines below pmat = periodogram_matrix (*dset->Z,dset->t1,dset->t2, -1, &err); for(int i = 0; i<2*pmat->rows; ++i){ std::cout<<pmat->val[i]<<" "; if(i == pmat->rows) std::cout<<"\n new row "; } std::cout<<std::endl; output the following 0.00617209 0.0123442 0.0185163 0.0246884 ...... 3.12308 3.12925 3.13542 3.14159 573321 new row 183464 89765.5 9105.27 30498.9 ..... 111.27 171.166 2481.53 559.825 200.179 288.666