RLC Low-Pass Filter with MATLAB
It is much easier to work in the s-domain and let Matlab do most of the work for you. See sample code:

L = 5000;     R = 1200;      C =  (10.^-3) ;

ZR = tf(R, 1);
ZL = tf([L 0],1);
ZC = tf(1,[C 0]);

H = ZC/(ZR+ZL+ZC);

bode(H)


Which yields the following bode plot:





EDIT:

I can't comment on the utility of negative frequencies, however your vector omega misses all of the interesting frequencies with respect to the filter.

Use the following span of test frequencies to repreduce a similar plot to Matlab's builtin in bode plot:

omega=logspace(-2,1,31);


A bode plot typically a log-log plot (y-axis is log due to use of dB).