Accidentals in Lilypond

sharp pitch add is to the note name.
flat pitch add es to the note name.
double sharp pitch add isis to the note name.
double flat pitch add eses to the note name.
half-sharp add ih to the note name.
half-flat add eh to the note name.

For example:

gisis1 gis g ges geses
accidentals

Quarter tones — series of A’ with increasing pitches.

aeseh1 aes aeh a aih ais aisih
quarter tones

Accidentals are printed automatically, but you can also print them manually. A reminder accidental can be forced by adding an exclamation mark ! after the pitch. A cautionary accidental (i.e. within parentheses) can be forced by adding a question mark ? after the pitch. Both ! and ? can also be used to forced natural signs.

cis4 cis cis! cis? | c c c! c?
forced accidentals