\version "2.15.0"

music = \relative c'' {
   % Ok, it works in Staff and TabStaff
  <b\2\harmonic>2 <e\harmonic>

  % it works on Staff only: bug or what?
  \harmonicsOn
  b2\2 e
  \harmonicsOff

  % harmonicByRatio
  <d\3\harmonic>1 % almost right, but I'd like to play the same harmonic on the 7th fret
  % let's try
  \harmonicByRatio #1/3 d\3 % TabStaff ok, now the pitch in Staff is wrong

  % harmonicsByRatio "breaks" following harmonics (one octave higher)
  <b\2\harmonic>2 <e\harmonic>
}

\new StaffGroup <<
  \new Staff { \clef "G_8" \music }
  \new TabStaff { \clef "moderntab" \music }
>>