For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:kosht/openSUSE_Tumbleweed/home:kosht.repo
zypper refresh
zypper install ngspice
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:kosht/15.6/home:kosht.repo
zypper refresh
zypper install ngspice
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:kosht/15.5/home:kosht.repo
zypper refresh
zypper install ngspice