Assigning values in a vector in non-sequential order

I want to implement a code similar to the below-mentioned one using vectors in the most efficient and easiest manner. How do I do it?

int a[1005];
for(int i=1;i<=1000;i+=5)
    a[i]=i*i;

When I try doing the same using vectors,compiler throws assertion failed error..subscript out of range...

vector<int> a;
for(int i=1;i<=1000;i+=5)
    a[i]=i*i;

So,I tried pushback function...However the index of vector could only be increased from 0 to number of calls to pushback and not in the way I want i.e 1,6,11,16

vector<int> a;
for(int i=1;i<=1000;i+=5)
    a.pushback(i*i);