Without sample input and expected output we're just guessing but MAYBE this is the right starting point for your script (untested, of course, since no in/out provided):
#!/bin/bash
awk -F, '
NR==FNR {
file = $1 ".pdb"
ARGV[ARGC] = file
file2col2s[file] = (col1to2s[file] ? file2col2s[file] FS : "") $2
next
}
FNR==1 { split(file2col2s[FILENAME],col2s) }
/HETATM/ {
for (i=1;i in col2s;i++) {
col2 = col2s[i]
if ($0 ~ col2) {
sum[FILENAME,col2] += $7
cnt[FILENAME,col2]++
}
}
}
END {
for (file in file2col2s) {
split(file2col2s[file],col2s)
for (i=1;i in col2s;i++) {
col2 = col2s[i]
print sum[file,col2]
print cnt[file,col2]
}
}
}
' massbuild.csv