CHARMM TIP3P

you have to. how do you think fix shake would otherwise know what

your desire bond length or angle would be?
Yes, thanks.

since you are also running minimizations, you should increase the
force constants on both (at least during minimization, i use a factor
of 100

The force contant for bond is already 450 and perhaps I just need to increase the force constant for angle from ‘55’ to somethig other larger value during the minimization.

During NPT simulation, I just need to use ‘450’ and ‘55’ together with ‘fix shake’, same as CHARMM regid TIP3P water forcefield,right?

since you are also running minimizations, you should increase the
force constants on both (at least during minimization, i use a factor
of 100

The force contant for bond is already 450 and perhaps I just need to
increase the force constant for angle from '55' to somethig other
larger value during the minimization.

you don't have to, but it helps keeping the waters "in shape"
which makes it easier on the shake algorithm in the MD.

During NPT simulation, I just need to use '450' and '55' together with
'fix shake', same as CHARMM regid TIP3P water forcefield,right?

as i already wrote. those force constants have no meaning when using
shake on the corresponding bonds or angles. they are ignored.

axel.