tie collision with stem

[libero . mureddu3]

Hello list,
I have one problem:
in this example, at the 3rd measure, the tie on the "a" in the upper voice
touches the stem of the note; how can I tweak a tie? I tried to read the
lilypond-internal about ties, but I cannot really understand how to do.

Thank you in advance
Libero

<< \new Staff {
<< {a''1~->
                               a''2.~ a''16 g''\( fis'' e''
 \stemDown d'' c'' b' a' \stemUp g' fis' e' d' c'  b a g fis e d e\)

                              } \\
                               {r4 a'16 d' a a' d' a a' d' a8 a'16 d'
                                a a' d' a a' d' a a' d' a a' d' a8 r8
                              }

>>

   \new Staff {
c1 c1 c1 c1}
>>


lilypond 2.4.5
macosx 10.3.8
ibook g3 800